Slipstreaming Passing Vehicles
This seems perfectly fine.

Cyclists often stand up for various reasons, and there's no reason to think that it would startle drivers. They probably have no idea why you're standing up anyway. Some people may eventually realize that you stand up every time they pass you, but I doubt they'll be anything but mildly amused.

Actual drafting (riding behind a vehicle) can and does stress drivers out on top of being rather dangerous for the cyclist.

As to the speed effect, standing up will make you go faster if the air is moving faster than you are. Effectively putting this into practice has a lot of nuances though, and really should be a separate post.