What is the norm? How many of you use the subsonic filter with your turntable?

My preamp has one but I never use it. At one time I had some feedback and woofer pumping when I had dual 15" subwoofers, the filter didn't help at all. But moving the turntable took care of the issue. Most of the time location of the table and/or speakers will fix things.

I have a Hegel V10 phonstage which includes the filter but they are neutral about using it; and my dealer told me at the outset that he never found a need to use it. In the 3 months or so that I've own it I've left it off and haven't run into excursion to necessitate employing it. 

It depends.  That can drain a lot of amp power and can result in clipping and speaker damage  but if not going too loud and noise generated is not a concern the filter may not be necessary.  Best to address the issue at the source and minimize it to extent possible. 

My phono preamp doesn't have one. I guess I've been lucky as I never have significant woofer excursion on my 10" sub or the 6.5" mid-bass drivers. I am even using a DD table in fairly close proximity to my speakers. Maybe it's the concrete floor that helps. Perhaps you're experiencing some tonearm/cartridge compliance issues. 

I have a full-range speaker system (IRS Beta) and no rumble filter. VPI TNT III sits on a VPI stand. No rumble, no feedback.