You can definitiely do it afterwards, but it's a lot easier if at least the drill-fill bit is done before varnishing. In fact, I like to take my boats out for test rides before I varnish them just in case I need to change something. It really sucks to spend a few weeks on a nice finish, then have to redo it because something needed moving.

Have fun,

Laszlo

   Lazlo, I am also not that far away from the varnishing stage. Can you elaborate on the moving stuff before varnishing. I'm not really sure what you mean by that.

Things like oarlocks, padeyes, rigging, seats, etc. Anything whose proper function is position-related  and would mar the finish if it had to be moved elsewhere.

Laszlo

   Please dont  consider this weird but if you DFD after varnishing, the little ring of exposed epoxy around the final hole is exposed to UV.   Even though it's a very small area, is it subject to cracking or self-desctruction if it is in the sun?

   ^^

I wouln't worry about a little bit being exposed. In fact, in my experience whatever piece of hardware (toggle, tidedown, etc.) you drill and fill for is going to cover up any unvarnished epoxy.