Agencies typically take 40% to 50% of what you pay an agency lady. But you should tip the lady only if she provides good service. Unless the agency is a bad one, the money that you tip the lady is her's. You should not feel any pressure to tip, if a lady is good and gain knowledge, she will move on to become either an independent or work for an agency that takes a smaller cut.

Not entirely sure what sort of "tipping" you are asking about so I'll cover both

1. Some less than reputable agencies will charge an "agency fee" that is simply the agency's cut for her showing up. Then she will have to hussle you for "tips" to earn her money. This is a scam by con artists that will never result in the servie you desire.

2. Good reputable agencies who treat their ladies right will split the fee 50/50, 60/40, etc. Since she has made an agreement with the agency, I feel it is up to her to negotiate a better deal if she is not happy with whatever income they can offer, just like you and I must ask our boss for a raise/promotion. To answer your original question, yes it is considered acceptable to offer a gratuity to these ladies if you feel you would like to. I think the general consensus on etiquette is to place the additional cash in the same place where you left the donation envelope.