I still party like a rock star on the weekends but I'll admit that I try not to stay out past 2am. If I stay out until 2am, I end up sleeping until noon and then I'll have trouble trying to get to bed around 11pm on Sunday night when I have to be up at 7am on Monday. There's been quite a few occasions (like 3 times this month) where I've been out until 5am.
I felt like crap by Monday and I think that's why I caught bronchitis. My body hates me right now.
As long as you're responsible and you're still going into work on time and getting you're job done, I see nothing wrong with partying it up on the weekends.