i have to agree with occasionalhobbyist. you give your point of view and others disagree and suddenly we are jumping down your throat? if you don't want to know what people think don't start a thread.
I see a huge difference between tipping and negotiating for extras. however, and i've never seen you so i may be wrong, i hae a sense that if you were to ask me for a tip "upfront" i would give my standard answer, that i do not tip before a service is provided. you should bear in mind that this is not something i have always done, but a lesson that i've learned, that in my experience i never, not one time, felt like i got my moneys worth when the tipping was done in advance, and also felt like the tip was totally unappreciated. i also sense that my refusal to tip upfront would then put you in a sour mood, and the whole session would be affected. after a crappy session, i am not likely to tip, because i have already paid a lot of money and got a substandard session.
you on the other hand might think that you are glad you were mechanical etc and did not put any real effort in because i did not tip to your expectation.
now again you've never seen me, and i will admit i am not a frequent hobbyist, but when i decide to see a provider i ahve no problem paying or tipping, and certainly would not expect to get some free extras.
i think however that you should be honest and tell a client up front, if i don't get xyz tip up front then the ramifications will be abc. be honest with yourself, what are the ramifications of refusal to tip up front, then tell your client.
as stated above god gives you two ears and one mouth so listen twice as much as you talk.
good luck, i think you will need it.