I've had issues with paint wearing away if handled in the same location, so I'd rather varnish than not.
I used to swear by dullcoat, I have a sizeable amount of models that received a layer of it and they all look/feel great with no noticeable discoloration. That has unfortunately changed in the last couple years, as every can of dullcoat I have bought has done little to matt the finish of my models. I get most of my hobby materials from a model train store and the regulars there say (and from what I've found on various forums) is that Testor's has changed their formula due to importing reasons. I've recently tested dullcoat yet again in addition to numerous other varnishes on about 20 test marines. All the marines were airbrushed with Vallejo primer and left to cure for a week and then airbrushed with a zenthal coat of red (
BA style) which was also left to cure for nearly a week. Lastly as I use oils, all 20 were given a coat of Testor's glosscoat which is also useful for determining the effects of each of the forthcoming varnishes.
I tried two cans of dullcoat, one that was purchased about a year ago that did not give the finish I was expecting, and a second can that was bought new. Both cans were shaken very well, and sprayed indoors under a vented hood at room temperature, this is actually where I tested all the varnishes. Both cans gave me only a very slight matting, it wasn't even satin.
Next I tried Testor's Lusterless spray from their model master series. (
http://www.amazon.com/Lusterless-Flat-Lacquer-Clear-Testors/dp/B004QPCLQG). This had absolutely no effect on the models. It was so bad that I'm planning on going back to the hobby store to exchange it as it must be some sort of fluke... they were that glossy.
Next I tried Vallejo's matt and satin varnishes in an airbrush, once each straight, once each mixed with some water and once each mixed with Testor's acrylic thinner (will get to that below).
The satin gave me a frosty finish each time, low PSI on the compressor, went on smooth, couldn't tell you why it didn't want to co-operate. Now that's not to say it was frosty like some of the posts occasionally posted here on Dakka but frosty enough to not get a pass. The matt varnish was much better but still had a little bit of a frost to it. Vibrant reds seemed dulled, and depending on the method, a little bit of frost could be seen. Both appeared to have a better result when used with the acrylic thinner.
Lastly I tried Testor's Model Master Flat (
http://www.hobbylinc.com/htm/tes/tes4636.htm) thinned with the acrylic thinner I mentioned (
http://www.hobbylinc.com/htm/tes/tes50496.htm). This was by far the best of all the varnishes I tried. The best result came from a 3:2 ratio, thinner to varnish. That being said, it still wasn't quite what dullcoat used to provide, but it was pretty close.
I've heard of people having good experiences with both golden and liquitex. I hope to test them soon.