Is it safe to leave a SLA battery connected to 12v all the time for "trickle charging"?
A trickle charge "float voltage" in the range 13.5 to 13.8V is safe (and even healthy) to be continuously applied to a typical 12V lead acid battery.

Anything below that range won't effectively keep it charged, and above that range may cause long-term degradation.

Make sure the buck converter has adequate filtering - you might consider an extra LC filter stage on its output - so that the battery doesn't see ripple or switching transients that exceed 13.8V.