A tune-up wouldn't be a bad idea, but its hard to say how much you should pay or how badly one is needed without knowing exactly what they plan to do. "Tune-Up" has kind of become a catch-all phrase for a lot of different, but fairly routine, repairs. If you do decide to get one, make sure you find out exactly what is going to be done.
Personally, I'd take a look at your car's manual, and find out what work is scheduled to be done at or around your current milage mark. There's generally a pretty extensive list of things that should be done at the 30/60/90 thousand mile marks. If you are approaching one of those points, or have recently passed them without doing those repairs, I'd recommend taking care of them before heading out.
Otherwise a good basic pre-trip inspection, taking a look at your belts, brakes, fluids, etc, is a good way to find out if you've got any other issues that could cause problems on your trip. If you aren't comfortable doing this yourself, you can usually have it done for $20-40, or some places include a inspection with an oil change. However, if you are getting the inspection done with your oil change, I'd make sure it is being done at a reputable shop, and not by some teenage greese monkey at the Quirky-Loob.
Happy Trails!