Roses are traditionally pruned in early spring.......when the forsythia is blooming. And it will not hurt to prune them hard. As far back as you like :-) I once cut a big, rangy rambler/climber back almost to ground level and it was full and lush and nearly back to the same size by the end of the season.

Whether or not a hard prune will stimulate budding from lower on the canes is a bit of a toss up. Old, very gnarly grayish woody canes are unlikely to produce low buds....younger canes might. This is pretty typical for many large growing roses - they tend to have 'bare legs'.

If you routinely fertilize your roses with an alfalfa product, you are very likely to get new stem growth from the base - what is known as basal breaking. This is often the best way to rejuvenate old leggy plants as once these new canes begin to mature and produce, you can remove the older, bare ones.