If you have weather for a destination and a suitable alternate and sufficient fuel to cover both then it's my understanding that you are fit to fly. If you decide to change your destination whilst airborne then, provided you have sufficient to cover the above in respect of your new destination, it is OK to do so.
Where some skate to the edge of legality (and most certainly beyond the edge of airmanship) however is where they fuel plan & flight plan for a destination, eg Kuala Lumpur to Paris then 'divert' to London where they had planned to go all along, landing on minimum(ish) fuel. One such example at LHR in the recent past had so little fuel on landing that they asked to be towed from the taxiway onto stand!