I have rules for opening lockboxes for people. I am a dutiful rogue (see greedy bugger that always picks pockets) and keep up my lockpicking skill, so I never get skill ups from other peoples lockboxes, unless they catch me just after I've levelled.
1. Guildies should never give me anything to open a lockbox. In fact if they want to send me loxkboxes through the mail, I have no problems opening them and sending them back.
2. If someone messages me, asking politely, I will never turn them down, and expect nothing for it. Sometimes I like to ask what they got, because I'm curious.
3. If someone is not polite, or messages me and then opens up a trade window without waiting for me to reply, I will ask them for a tip. No tip, I will not do it.
4. If someone just opens a trade window without asking first, I will refuse. These usually occur in the bank or in the AH when I'm in the middle of something. That bugs the heck out of me.
5. When people ask in the trade channel, I will normally reply, because I can. I don't expect anything, but it's nice to get something back. I've found the most common tip to be 1g. I've gotten 20s, 50s, 1g, 2g, and 5g once. That was pretty sweet.
6. I always thank the person if they give me something.
I know it's probably a lot ado about nothing, but those are the rules I follow.