Hello Paul,
I've always been reluctant to use varnishes, particularly for the sake of decals, and this even for my airliners.... Past experiences and numerous disasters, particularly with stuff in cans, have made me that way!
Flat colors are easier to apply, I find, and instead of risking twice seeing my finish not ending up like what I want, I've come with the 'technique' of applying my decals onto fresh and still wet patches of Future floor varnish. This seem to drastically reduce the 'silvering' effect, those pesky air bubbles that are trapped between the paint and decal. If you can get your hands on decal setting solutions, the combined use of Future and decal solutions will do the trick at 98%. For the remaining 2%, which you'll see when the decal is dry on your model, use a needle to pin-prick the decal and put some more Future or decal solution.
Normally, a normal layer of Future will not change the appearance of your paint (you know that gloss varnish tends to make the paint look 'wetter', darker. If it does another thin coat of Future will even things out and may give you a satin finish which is often closer to reality that flat finishes anyway.
As for your Spit, if you're happy with it, leave it as it is. If you are not, take the risk with a matt/satin varnish.