Is there a "standard" height for towel bars in bathrooms?
I believe it's 48" for a bath towel. 

I just put up new towel bars in my bathroom, and my method was to hold the bar up with a towel on it and have my wife tell me if it looked right. Just to be sure, she even acted like she was drying her hands to see if it felt right too. If I remember correctly, it came out to 46". (We're both short - the next owners can either complain about it or raise it themselves.)