Small bits of rag aren't worth bothering about with soaking. It's large quantities of rags that have been soaked in the stuff and left all bunched up that can be an issue as the heat given off by the curing linseed oil builds up and as the rags are insulating, it gets hot in the centre. Fill a trash can full of the stuff and you can get fire. One small piece of rag on its own won't even feel warm to the touch.
But the clear poly spray is the safest method if you are unsure or can't test on a spare decal beforehand.
And I'd still put the decal on over at least one clear coat, otherwise you will trap some of the 'satin' finish under the decal, and any clear parts of the decal will show the finish below as a slightly different shade.