Well, to take the guesswork out of this, our ops manual include a deciphering table to see what needs to be taken into account and what doesn't, and for what conditions. With regards to a TEMPO in a TAF it essentially says the following:
For an improvement in conditions, disregard the improvement.
For a deterioration on destination, disregard if it's a shower, otherwise consider.
For a deterioration on an alternate, add 1 hour fuel if shower, otherwise consider.
This from memory and it's late, so bear with me if I mixed it up. It's in our quickref anyway
