I've never had a hotel ask to see a girl's ID in Bangkok. Maybe a nod from the receptionist, but that's all.
But if you really want to be able to skip walking past the reception completely, stay at the Doubletree on Soi 26. They have a back entrance to Soi 24. It's open 24/7, so you can enter and exit without going by the reception. It's very easy to find, just walk right out of the main elevators instead of left to the reception. It's also a shortcut to the BTS, so many people use it.
But as others have said, don't worry about it. The staff really don't care. Before my first trip to Bangkok, I did a lot of research about which hotels were guest-friendly, but now I just assume any quality hotel on Lower Shukumvit is. It's never been a problem.