10 stop filter focus

I have been using a 10 stop filter for a few weeks now for certain landscape shots of the sea. I am using a screw on 10 stop B+W ND which is excellent quality. I am LV shooting and to save taking it on and off again between shots I simply leave it on and have even (during day) been able to focus through it.

What I am doing is: initially at the scene metering, then reducing shutter by 10 stops.

Filter is then added to front of lens.

I go about my day finding shots, when I find one I

1. Open aperture up to f/2.8 on liveview (always ISO 100) and I can see easily through viewfinder to compose

2. Decide on relevant subject and focus (believe it or not it works through viewfinder - but I cannot focus when I am stopped down for obvious reason)

3. I then move to bulb mode and stop the lens down to f/8-f/22 (anywhere between) and take the shot

Now I am aware of what is called focus shift due to spherical abberation when a lens is focused wide open. My question is, does it matter when I am stopping down to such tiny apertures? My results seem to be sharp most of the time at 100% on the subject which I am focusing. Unless the sand moves or something...