Varnishing is a pain in the arse, but if you want to protect your minis you have to. Testors dullcoat is ok, and so is purity seal, and yes you don't need to lag it on. It also will not harm your base.
THE MOST IMPORTANT thing to take note of when spray varnishing is to do it on low humidity days, or you will get white crazing on the mini. If this does happen, a brush up with matt varnish will sort it out.
If you are after out and out protection, for say blood bowl minis, I use matt wood floor varnish. It does the job a treat, and if it's still a bit shiny, paint on some matt varnish