It really depends on how much you're riding and what your goals are. Personally, I like to do a lot of climbing, so I try to keep my body weight low. I do upper body lifting once or twice a week and it's nothing intense- just enough to try to keep my bone density high. I was always a hard gainer anyway, plus I ride too much to really gain any mass from it. If you're more into flat riding, a little extra mass won't hurt.
When it comes to legs I have a more seasonal approach. I hardly lift at all during the summer, but I'll do once or twice a week during the winter. When I hit legs I hit them hard, so it takes 2 days to recover. I'd rather spend that time riding during the summer. Winter is a time to sort of recharge the batteries and lifting is a part of that for me.