As a reference I mine zec.
I have 2 different setups and I actually get more stale shares on my hardwired home setup vs my remote wifi setup (different location).  Kinda weird but yes percentages are very low as in maybe 1% of those shares every few hours.  I can go days and not get a single one, and then sometimes I will get a few (what ends up being 16 at a time) during 1 10min block.

I've only noticed stale shares on my 1080TI of the few cards I do run.  Its the only one that does it.