Mike,
I have watched the increase occur while flying from cooler to warmer air, both on long trips with sizable changes in OAT or in descents from high altitudes.  On the flip side, I have watched the pressure decrease in rapid climbs to 18k, sufficient to cause the pump to recharge, then hold steady for the next four hours.  Another variable is heating of the nose gear hydraulic cylinders by engine cooling air.  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.

It would appear the cracking pressure of the relief valve is higher than its nominal setting.  If I remember, I'll do a bench test this weekend to check this out.
Chris Zavatson
N91CZ
360 std