The DFM service I use, flags anything less than 150mil (~= 4mm). The guys who run the service, say that's a worst case condition for their house process, which is to use wave solder (preferring full wave, with selective wave as alternative). The clearance is for the fixture covering the SMT components. So the clearance is actually a cone frustum around the THT pad. The size of the cone depends on the fixture technology used: milled fixtures are cheaper but thicker than, say, formed titanium sheet.
Or you can do a full-wave process (no reflow), which I think is going to be more common with wider pitch SMTs and high volume production.
Tim