Always apply a clear varnish coat. It won't stop chipping/peeling, but it will reduce it, and/or slow it's onset.
I painted tan and green Army Men figures, in that plastic which nothing will stick to… I used Minwax Polyurethane Stain, to seal them. It will peel off the Army Men plastic, in a few years, but during those first few years, it is golden!
If it works that well on that chemical-resistant plastic, how much better will it adhere to metal/Bones/resin figures?
Been varnishing my mini's for more than 20 years. I'll never skip the varnish coat -- I hate re-painting figures, so I keep it to an absolute minimum.
I use cheap craft paints, as well. If you are willing to spend the money on high quality paints, why skip the varnish to save a few dollars/pesos/whatever? The time saved on the front-end, will be lost when it comes time to re-paint, later… Cheers!