It's a balancing act. My 50-60 mile round trip commute is through the city, and getting up to speed after the many stop sign/lights is what seems to trash my legs out faster than anything. If I know I won't be riding the next day, I'll push it more in general, but if it looks like I can commute subsequent days, I try to take it a little easier. Today is Day 4, and my legs are definitely not fresh. I had a few energy drains from Days 1-3 I wouldn't mind getting back--pushing over a hill, not wanting to get caught by a cyclist behind me, chasing a green light, quicker starts than needed, etc. :-)