My rule of thumb used to be to ignore TEMPOs unless both destination AND alternate were TEMPO'ing below limits.
For what it's worth, there's a new AMC proposed in Part-NCO (non-commercial OPS) for interpreting TAFs for the purposes of aerodrome operating minima.
AMC1-NCO.OP.165 Meteorological conditions
It echoes the way that JAR-OPS1 used to do it. Essentially (and please, read the AMC before relying on these rules of thumb):
* During a BECMG period, you take the worst weather
* You can disregard any PROB30 TEMPO
* You can disregard a TEMPO related to transient/showery conditions
* You must take account of a TEMPO related to persistent conditions like fog
* You should always disregard a TEMPO improvement