As long as it's a minor leak, I wouldn't make it a HIGH priority, unless other work was being done in the area. When you do replace them, definately install full-flow ball valves. I'd replace the valve to the WH when I replaced the WH, and test the main shutoff now for adequate operation. (Depending on its environment, the main shut-off has a better chance of still providing a tight seal.) You can also use silicon plumbers lubricant to lubricate the valve stem before closing and opening it a few times, and also tighten the cap nut a little if it starts to seep around the stem.

There are basically three methods used to mount the angle stop valves. 1. Solder type 2. 1/2"NPT Thread mount 3. 5/8" compression mount. In new construction the angle stops are generally installed during the final trim out phase of the plumbing, which generally occurs after the painting or wall finishing phase. At this stage there is a very high risk of causing damage to the wall finish if we were to install the solder on type or to solder a thread adapter on the stub out. For this reason most plumbers elect to use the 5/8" compression type which only requires a simple mechanical connection. To install the compression type angle stops the stub out is cut to the desired length, then the compression nut is slide on the stub out, followed by the brass ferrule and finally the valve is slipped on. With the valve held firmly in its final location, the ferrule and compression nut is then slide out to the valve and tightened finger tight. Once the compression nut is finger tight the plumber uses two wrenches, one to hold the valve steady while the second wrench is used to tighten the compression nut. As the compression nut is tightened the inner brass ferrule is slightly compressed into the wall of the copper pipe thereby locking it firmly in place. When changing compression mount angle stops many people make the mistake of attempting to pull the ferrule and old nut off the copper stub out and usually in doing so they distort the shape of the copper making it very difficult to get a tight seal with the new nut and ferrule. A much simpler and more reliable method is to unscrew the compression nut then slide the old valve off the copper pipe leaving the original ferrule and compression nut on the pipe. You then remove the compression nut and ferrule from the new valve and slide the new valve on the pipe, tightening it in place with the original nut. While this method does not work 100% of the time, as a rule it will work most of the time and certainly bears trying before you begin cutting the pipe or trying to remove the old ferrule. When tightening the compression nut you can use a pair of channel locks to hold the valve steady, but when doing so be sure to pad the valve with a rag first so the teeth of the channel locks does not scratch the finish. I find an open end wrench to most convenient to tighten the nuts in a tight space. (I especially prefer my Craftsman ratcheting open end wrench.) When selecting angle stop valves I use the solid brass 1/4 turn ball valves exclusively. The ball valves have the advantage that they are quick acting full bore valves and they are also have the advantage that when turned they are self cleaning which is a distinct advantage in areas where mineral scale builds up on internal valve parts. I will probably open a real can of worms here, but My first choice of supply lines is chrome plated brass tubing with the cone washers that come supplied in your faucet set and the compression nut and ferrule that comes supplied with your new angle stops. I have seen copper or brass tubing supply lines that have been in continuous service in commercial applications for 20 or 30 years and they still work fine. My absolute last choice would be the stainless steel brain lines. While the stainless steel braid gives the illusion of being some kind of high tech space age strength and durability in the final analysis many of those lines are the absolute weakest link of all the choices. It is true that the stainless steel braid will prevent a line from blowing out, but when we consider that the maximum allowable line pressure is 85psig, the chances of any lines actually blowing out is slim to none. On the other hand, what most people don't know is that a vast majority of those stainless steel braid lines has nothing more than a stainless steel braid over a cheap neoprene hose. Under certain water conditions neoprene has a relatively short service life as is evidence by the failure rate of neoprene toilet flapper valves. In regions where they have problems with the water deteriorating the neoprene I prefer to use chrome plated brass lines with the factory preformed cone washers that are made out of nylon.

Where exactly is it leaking? If it is at the stem of the shut off valve the you could try tightening the packing nut. Be gentle and don't force it. If leak does not stop you might be able to replace the valve packing. Otherwise the valve will need to be replaced. If it is coupling between the valve and the radiator then you can also try to tighten it. If it continues to leak you will have to disconnect it and put in thread and pipe dope to make a better seal. There may be material specifically for steam piping but I'm not sure what it's called. Don't forget to shut off the valve if you disconnect the radiator.

The handle is only held by the screw. If you replacing any internal parts you will have to shut the water off. You can try and tighten the nut and see if it stops leaking...In many cases when they leak you have to replace seat washer, but if you going to do that you might as well replace the entire stem they are not expensive.