I have done both with lettering on My semi . Spray and non-sprayed . Have used water in a windex bottle and also windex . Plain water does not work good , as it tends to linger , were as the windex , evaporates , thus you have a Window of time to get it right before full contact . Basically what you are doing is just delaying the contact time to squeeze out the air bubbles with a stiff card , be it license , credit card , or other small straight edge .
Without the delay , air bubbles are a lot more difficult to get out , other than the needle trick .
Have never had a issue with decals later peeling off . Current truck has had them on there foralmost 4 years . Previous truck had same decals for over 7 years . When it takes a heat gun and nimble fingers to get them off , Have to be doing something right .
Fred H..