It depends on the bike...
My 62 Pug has 11.5 inches of clearance so running 170's is no problem at all as if I strike a pedal I figure I am probably already screwed...and I push this bike really hard.
Some of my other conversions run lower clearances and I don't run anything less than a 170 crank so I have to be mindful of what I am riding and how hard a turn I can make. I have two old Kuwahara mtb's that have been converted to fixed gear bikes that actually have less clearance than the pug.
I use one of them for messenger work (it has 170's) and have not had any issue with pedal strikes but know it cannot corner as sharply as my pug (which is also seeing this kind of work). The other runs 175's and is used for commuting / touring and I have yet to strike a pedal.
It all comes down to rider skill and knowing the handling limit's of one's bike...since I also ride geared bikes and rarely coast I have to know when I am about to cross that line but on a geared bike the consequences are far less severe.
This bike has very low pedal clearance but is still so stable and manoeuvrable from a fairly upright position that I have yet to strike a pedal.