Don't get me wrong, if you're just going to add some nice oil and black pepper and some nice hard cheese--parmesan or pecorino or such--and maybe a fistful of herbs then oil in the water probably isn't a problem.
Now, whether or not you rinse your pasta after cooking is a trickier thing. I generally won't unless I'm going for something like baked Macaroni and Cheese, where the extra starch isn't really needed with the bechamel sauce.
Bechamel tip: use Ghee (clarified butter, often found in the Indian or Middle Eastern section of larger grocery stores) instead of regular butter. It melts so well, and the flour just dissolves into it so much nicer. I've had a lot fewer lumps making bechamel sauces with ghee than regular butter.
Edited by theBitterFig