Never considered tipping service writers or mechanics. I figure at $75 to $125/hour, they should be tipping me.
Now I know the mechanics don't get that total but still, that's highway robbery and throw another $20 on top of that for tipping? Last time a service department (correction, an HD service department) performed service on my bike (I discovered all of this when I performed the next service) the oil filter was tighter than dick's hatband and three of the 5 screw heads for the primary service cover broke when I attempt removing them for oil change. I had to drill and tap to get them out they were so tight. That 30 minute routine service took me a day and a half because of the dealer's service dept.
Glad there was no tip involved with that service...

I did use the service department for the first, 500 mile service on Cochise. They were $200 less expensive than the first HD service AND they let me watch them. They did everything on the list and were very friendly when I had a question. Still, no tipping involved.
When you perform you own service, where would the tip come from?...

By the way, to answer the original posting, I wouldn't pay for ANYTHING until the requested work had been completed, we (me and the mechanic and/or service writer) inspect for proper install and equipment, they had the old parts for me to take home (I would have requested that up front) and it was at least as clean as it was when I brought it in. If it wasn't as I expected, it would get reworked and problems corrected on their dime.
At that point, payment...still no tip.