Not all wheel tunnels are blocked off around the nose gear.  Even those that are, may still allow a lot of warm air into the wheel well.  On a typical flight one might see two cylinders get warmer and four cylinders get cooler. 
I would bet a lot of these factors cancel each other out most of the time.

Chris, Et Al,

 

Yep, this is also why I see that my nose gear strut pushes the nose up higher on landing after a long flight - Nitrogen is affected by temperature - see Boyle's Law, or was it Hoyle's Law? - I guess I just played too much bridge during my Chem 101 era.