Welcome, Brandon.
A lot depends upon how you construct your showers, but I wouldn't do it with epoxy grout.
Those of us who have violated the rule about flexible sealant at changes of plane in our showers for years do find that sometimes the cementitious grout cracks. If it were an epoxy, chances are it might crack the edge of the tile instead. 'Specially with glass or other weak tiles. And not sure why you'd wanna do it on the vertical corners but not the horizontal corners.
All you can do is try it if you really think it's a good idea. Might be years before you find out how much of a problem it might be, though.
As for matching the grout with a flexible sealant, you might wanna talk with the folks at
ColorRite. They can generally make you a 100 percent silicone sealant to match any grout color on the market. And you can get it in a sanded or satin finish, too. Might solve your whole problem.