There is a difference in the dispatch fase and flight execution! EASA, During the dispatch fase you need taxi out, trip, cont, alt and final reserve, if dispatching without alt(allowed under certain conditions) the final is 45 otherwise 30 minutes.
During flight execution( from start pushback or moving under own power) the only regulation is to land(anywhere) wiith 30 minutes remaining!! If anticipating to land with less than 30 minutes than an mayday call is required.
Again, under EASA, there is no requirement to land at destination with alt fuel plus final reserve! I believe tha FAA rules are the same.
at some point you are going to commit to a specific aerodrome, be it the destination or alternate. It is up to the commander to decide which is the safest course of action. What is safer? Coming into your destination with alt fuel and final and then having to make a GA because of, say for example, traffic still on the runway, then committing to your destination and using your alt fuel and being able to make a few approaches using that fuel, or diverting to your alternate and then having to land immediately before using your final!
Remember also that as captain you may always deviate(not applicable in this particular case) from the rules in an emergency or to avoid an emergency( in the interest of safety)!