I can't speak about standard practices, but in the  dozen or so museums
where I have installed exhibits  all glass exteriors of cases have over
about 2 square feet have been tempered glass over the last ten years or
so.  Before that I used a lot of double strong glass and never experienced
any damage from breakage during an exhibit (although a good deal of damage
was caused during shipping).  A lot of plastic is used because it is easy
to work with and generally  safer but of course it will mark much easier
than glass and will burn. Tempered glass kinds of ties your hands because
it is difficult to work with  and  kind of forces you into standard size
pieces.  There are several factors regarding glass vs. plastic form
framing I let a specialist answer that.