I prefer them to the bottom of the trim. I do like them when they go to the floor, but not for a sewing room, unless you have very high ceilings.

I do think it's a matter of taste whether you want them to the sill or covering the apron.

How are you hanging them? If you're using the spring tension rods, that fit inside the window frame, I think it looks better for the curtains to just go to the sill. If you're using rods with brackets attached to the side woodwork, I think it's really up to you--which look you like.

I will say this, in our new house, the original builder was a tile man, and we have a lot of real marble window sills--as you can imagine, I do NOT cover those up with curtains that extend down past them. So that would be another factor to consider--how good the sills look.

My suggestion? Why don't you cut the curtains long enough to extend past the sill, hem the bottoms, then hold them (or get someone else to, so you can actually get a decent look at them), and see which way you like them before sewing in the rod pockets? You can then make your decision

One other thing to consider if you're installing new brackets. Even though you're making these curtains, and can make them any length you like, install the hardware so that in future, you'll be able to use ready-made curtains. Normally, ready mades come: 24; 30; 36; 45; 54, 63, 72, 81 (or 84--depending upon the manufacturer)" long. Not all styles come in all those sizes, but those are the possible lengths for readymade curtains. I suggest installing the hardware at the right height for readymades, because if you (or a future owner) decides to go that route, it's better not to end up having to move hardware, leaving holes in the woodwork, obviously.

Have fun with the project. I've made curtains for just about all the rooms in the new house, and really enjoyed doing it. Did find one readymade pair for the sewing room that was a perfect color match AND had cute flowers and bird houses on them--they coordinated well with the border of quilts hanging from tree branches. BUT they only came in 36, and I really need to shorten them. I hung them (using a spring rod, so I could hang them high), but I need to get around to cutting a little off the top, so I can see out of the windows better.

I prefer them just to the sill. For drapes, I like them longer, but not curtains.

It might make a difference as to whether they are going to be hanging down straight or swagged over. Also the style of the curtain or decor of the room.

Can you describe the fabric, or haven't you bought it yet.

I just do what ever I feel like for each room, and don't care one iota what's IN. Personally I like things pretty plain and I avoid the heavy drape and layers look.