I have a older double trailer made for machines twice the weight of the sparks! tied down on the front at the winch post and at the back of the trailer, no issues with bouncing around, the older machines didn't have much of a problem with bouncing because they were so heavy, but I have seen them come off the trailer before because they weren't tied down also, it just takes a bigger bump!
not sure about laws where you are, but around here you can get charged if you don't have the back of boats/pwc's strapped down, if you did have one come off because it wasn't strapped down or didn't have heavy enough straps holding it you would likely get charged with an insecure load!
p.s. bungie straps don't cut it!! they should be load rated straps!