I just finished a small part of a huge collection, I charged the owner the same amount for each car done (some were easier and some harder so it evened out my per HR/rate) I was paid in full and cahs for all the cars I detailed + the price of another car (if I did another) as tip. Very very very generous!
I Never, never count money in front of a client!!! I counted it at home and immediately sent off an email thanking him for the opportunity and his gratuity. Dealing with someone who takes care of you like that and is such a powered powered person it would abosolutely offend them to offer back a gratuity, no matter how much the amount.
You need to think hard about what type of client and how he or she chooses to tip you.
Some people tip as a gesture because they think its proper but may feel its forced to services rendered. I personally think a detail is enough payment in itself if you charge exactly what you needed too.
If a customer hands me extra tip as a small gesture I politely refuse 1 time, if they persist I politely accept.
DO NOT GET INTO A DEATH MATCH TRYING TO REFUSE A TIP, YOU WILL OFFEND A CUSTOMER.
Reading people is key, see how they respond to offering or giving a tip. If you see true genuine appreciation (voice, facial expressions) then in their minds offering you a tip is 100% justified and they WANT you to have it for a job well done.
If they are slow to offer a tip or fumble for cash or a check than they most likely are stalling for time in hopes you say "Oh please no gratuity is needed".
Call me crazy but this is how I do things, hope it helps you.