I don't think there's any negative effect on the product, but I personally have the negative effect of having to chase it down when it's rolling under the vehicle.
i didn't really think it had a negative effect sounds retarded i know, just the big rubbermaid contianer i hold everything in is not tall enough to stand up my aerosol cans like my IG and power out carpet cleaner etc.. everything is squished in there tight enough already
Actually, I think the issue may be that if the can is on it's side, the propellant may get into the dip tube, which may leak out easier than the liquid contents leaving you with a flat can.
As long as no propellant leaks , no worries ! Usually the propellant is chosen that is benign while in contact with the product. So, there should be no adverse effect inside the can regardless of position.