I've got/had 3 - none have had a headstock strap, so assuming your magnets are up to muster and you aren't carrying housebricks in there, I'd imagine you'll be fine.
Done many miles with an Oxford tankbag on my CBR without any problem but first time I used it on the GS it came off with the wind blast from a passing truck
If your tankbag has a strap probably best to use it
always use head stock straps, takes seconds to put on and saves you looking like a T**T from trying to catch the tank bag as it flys past you on the motorway