Don't forget practicality and reality. In the UK, for example, the TA is usually around 6000ft. If you've been cleared (passing 2000ft) to FL80, why wait to set STD? Is it more likely that ATC will suddenly re-clear you to A6000ft, or is it more likely that you'd forget to do it (distraction, whatever) and never make it to FL80. If ATC re-clear you lower, that is their mistake and they should reasonably expect you to take a few seconds (reasonably more than a few) to reconfigure the FCU for the new clearance.
WRT low transition altitudes, going into AMS, you will get cleared down to FL45 and maybe even lower. It's no big deal, you just fly it. On a procedural arrival/approach, more attention must be paid to the TA and it's significance should make up part of your brief, other than that, it isn't a big issue.