It depends what your objective is and what painting you plan to seal with varnish and where you plan to display it.
Glossy varnishes - bring the subtle colors back to life, as with drying colors dull a bit and adds some playful depth. I use only this type.
Matt varnish - will change nothing to the dried painting and will keep the sunken colors sunken.
Satin is in the middle - I am hunting to get this type of varnish and compare to glossy ones. I think this type is a solution for all the problems of color brilliance restoration and glare removal.
The little trick with glossy varnishes you need to make sure where the lighting goes in a display room as artificial lighting is a killer if glare cannot be managed.
I guess if you are wondring about varnishing - you already gave 2-3 months of drying time for a thin layer oil painting. It is critical as if oil is not dry it will crack and what not. To preserve masterpiece for more years to come varnish only dried paintings. General recommendation about varnish is to do it after 6 months after finishing touches.