When I was a young man, many years ago, I was told by old veterans never lay an amp on it's side. Later on when started fixing amps for myself and friends I found out the reason. All of those old Fenders had big transformers and the lugs of the trannies were bolted to the chassis. What fender used to do on some amps was solder wire grounds to the lugs. When the amp gets layed on it's side the weight of the transformers would crack the solder and cause a bad ground and make the amp howl to high heaven. Just fixed a Fender Super 6 with this problem last month.
The weight of the trannies are the only problen I've encountered with amps layed on their side.
To this day I do not lay amps on their side when hauling it in a trailer on a long run, but won't hesitate thowing it on it's side if it's just across town and I know it does not have that solder problem.