What goes around comes around. I don't do favours for people based on whether or not they'd do the same for me if the boot were on the other foot, but my own perception of people's respect for me is usually based on whether or not they try to treat me as a doormat.
I often do go out of my way to assist people without reward, and I don't have a problem with that, but I'm more likely to do that for people who I think or have proven that they would do the same for me.
In the workplace I try hard to maintain a balance between being seen as a decent hardworking bloke who'll do a bit extra and being the mug in the corner who can have anything dumped on him. There are no hard and fast rules to what I'd do/not do, so it's difficult to put anything concrete into writing, but Sam's lab rat analogy is sound, it's just up to you to decide when to say no.
Relationships are a very difficult matter to consider in this context. My girl is often the person who's doing the cooking, putting the washing machine on, or coming to find me to bring me something I've been too stupid to take with me, so I would have been quite happy to interrupt my evening to fetch her from somewhere and would have just accepted that circumstances had changed if she then didn't need me to. I guess that's because our relationship is give and take all the time. I don't know how your's works, and you're not me, so I guess again I'm left with saying "you should do what feels right to you" again, even though that does sound pants.