It would probably depend on the situation and the installer in question. I never worked as a Cable Guy but I did do home theatre installations. People would ask if I would do this or that for extra all the time. Whether I said yes would depend on the person (were they nice?) the house (Messy, clean), my schedule, how my back was feeling (40" Tube TVs are heavy), difficulty and possible reward.
It doesn't hurt to ask. I would suggest throwing out a $ amount or asking what they would do it for. I doubt they would risk doing the work if you only give them a vague term like 'tip' as people have different ideas as to what is reasonable. Being nice, have a clean house and any equipment needed handy will up the chances of a 'yes'. If they say 'no' respect their decision. The more someone pressured me to do it the more I was determined to turn them down. Some people just don't know what 'no' means
