I wouldn't go light tent route. I have one of those and it works well for jewelry and little things, but unless you have a lot of lights, you aren't going to get the lighting you want to make it work in the tent. I assume your client has stands for the guitars. I would use those when possible.

What do you have for lights? What are you using for a background? Diffusers? If you provide some specifics of what your shooting with we might be able to give you some ideas using what you have to make them pop. The suggestions are already pretty good, but I think they can be fine tuned knowing what your working with.