The single Fn key on ANSI HHKB Pro is terrible, regardless of ones dominant hand. For example, I find it nearly impossible to press combos like Ctrl-F8 (=Ctrl-Fn-8), and I have ± average hands.
There's a DIP switch, that remaps the left Super key to Fn, but then you lose that Super key… The HHKB Lite layout (w/ an extra Fn key below left Shift) is so much better in this regard.
That said, the original HHKB layout didn't have _any_ Fn key at all (and Meta instead). No big deal, if you use only *nix shell, but most people don't do that.
Personally, I'd rather get a HHKB JP w/ hasu's replacement controller—because the big "ANSI" spacebar is a waste, and I'd rather make my own layers with more stuff than just arrow, nav and Fx keys.