Though I'm an enchanter, I've never advertised enchants - maybe I should start doing so, since it might be an easier way to skill up (I'm only about 200 or so). Since I am working at trying to level my skill, I wouldn't expect a tip if they supplied their own mats, but of course a little gold toward the mats needed for my mount quest wouldn't hurt.
Around the time I hit lvl 45 or so, I had my first request to use my warlock summons. I'd never used the spell before, and didn't even have it on my action bar or know exactly how to use it. (I solo most of the time.) The group offered to pay me 5g for the summons, but I refused the gold. I was just excited to learn how to use the spell.
The same day, I'd been wandering around Tanaris killing off pirates and was about to leave the area when a lower level mage asked me to help him with his pirate quests. I was in the mood, and had nothing better to do, so I helped him kill off a bunch of pirates, and then we both went after the assassins, rogues and such around the water fields. Just as we were about to split up, he took a look at my armor and weaponry and ended up giving me a bunch of enchants (his enchanting was much higher level than mine) which he said would have cost me 5g. I didn't have a lot of coin to tip him with, but I had a really nice green item which I couldn't use and didn't have the skill to disenchant, so I gave it to him, and lo and behold he actually ended up getting a shard out of it.
I tend to work on the supposition that what comes around goes around. If I'm nice to them, maybe somebody will be nice to me.
That said, if I were to seek out someone and ask them to take the time out to help me with a lockbox, enchant, etc, I'd expect to tip them. I'm glad you asked this question, because I have 2 lockboxes sitting in my bank, but I haven't asked to have them opened because I didn't have the foggiest what a fair tip would be.