If you really need to dryfit, I would suggest the K-Y. It is water soluble and could be washed off before permanent installation. Oils and vaseline would/could leave residue that will interfere with adhesion. Best is still to measure the pipe and fittings.

 

Your right, it is. But when you have to do 4 odd angles and drops, its hard to get it right on the button. Does it matter if the pipe isnt pushed all the way into the peice, im not saying i would just push it in 1/8 into it, but i mean about 3/4 way into the fitting

 

Since it not a question of length dimensions but angles, you should be able to slip the pipe and fittings in far enough without any lube and then mark the positions on the side or the pipe and fittings with a Sharpie marker for assembly. If this doesn't work, go with the K-Y. Good luck on your project.