Is airframe heating a concern at subsonic speeds?
Yes, aerodynamic heating is observed even at speeds that are well below Mach 1.

The practical application of this heating is the difference between Total Air Temperature, TAT, and Static Air Temperature, or SAT.  In essence, SAT is the actual temperature of the air, while TAT is the temperature that the moving aircraft "feels" due to aerodynamic heating.  When climbing and descending at low and medium altitudes, ice can accumulate on an aircraft when the TAT gets close to or below freezing. A common rule is to turn engine anti-ice protection on when in visible moisture (rain, snow, cloud, etc) once the TAT is below +10 degrees Celsius, since some cooling may occur in the inlet and you want to have a margin of error.

It is common to be descending at, say, 300 knots, in IMC (i.e. in clouds) and observe a TAT above +10, then when slowing down to be below 250 KIAS below 10,000', the temperature drops several degrees so that anti-ice is now required -- the difference being the additional heating that is lost as the aircraft slows down from 300 knots to 250 knots.

At high altitudes, roughly 30,000' or slightly above, where the TAT is always well below zero (although still above the SAT), a different limit is noted: below -40 degrees Celsius, the air itself is too cold to have enough moisture to cause icing, so in climb &amp; cruise below -40, the anti-ice can be turned off.  (In descent in IMC, the anti-ice is turned on regardless of the SAT, because descending into air that CAN support icing without your ice protection turned on could make your day far too exciting -- in a bad way!)

Also, at very high altitudes on a cold day, it's possible to have temperatures that are low enough that the freezing point of your jet fuel can be a concern.  Again, this is a matter of TAT, because that's what the structure of the aircraft feels.  One solution if the fuel tank temperature begins to approach the freezing point it to increase speed, which will raise the TAT somewhat.  If that's not enough, then descending to warmer air is about all that's left.

Bottom line, at sub-sonic conventional jet aircraft speeds, aerodynamic heating is very much a factor.