I don't know what others do, but here's what I do.

If the cashier has the desired strike readily accessible and doesn't have to go searching, I tip $1. If he/she has to look in the back, or look through a rack or two of strikes, I'll tip $2-3. I'll tip more for extraordinary effort. Once Dennis Finn and I both wanted a large number of a certain series of obsolete strikes and it took the person quite awhile to locate that many. We each tipped $5. Another time a cashier at Sierra Sid's was kind enough to walk with me out to her car to show me her special Sierra Sid's ET one pound strike (LTD 52). This was not easy for her, as she had a limp in her walk, which I didn't realize till we started to walk out. I gave her $5 for making the effort, even though I wasn't looking to buy it. I just wanted to see what this beautiful strike looked like.

In every case, the person was helpful and thankful to receive the tip. It didn't seem like they were necessarily expecting a specific dollar amount as tip, or even a tip at all and they usually put it in a tip bucket at the cashier that I expect was shared amongst the staff. In fact, after tipping at Sierra Sid's last weekend to trade for one of the new Western Series, the cashier double-checked with me just to make sure the dollar I left on the counter was a tip and not left accidentally! I'm sure this kind of practice by all of us promotes their cheerful cooperation when the next collector comes along looking to buy/trade for a specific strike at the cage.