I always give a girl gifts once her satisfaction is low enough that I won't lose any points of satisfaction.
eg. if a girl wants make-up, which adds 26% to satisfaction, I wait for the girls satisfaction to be below 74%. At 74%, she would get to 100%. At 75%, she'd also go to 100%, so you'd lose that % point if you gave it to her then.
Also, check everytime you log in and give your girls gifts if they need them. I had gone to prison because I sent a girl to a client when she was at 32%, and the lower her satisfaction is, the more likely it is you'll be thrown in prison and sent to the hospital. I think it's also possible that she will runaway or make it easier for someone to steal her from you, if she has low satisfaction.