Will a 3/8 roundover on the front baffle make a noticeable difference in response?
Most folks put a 3/8" roundover or larger on the front vertical edges. The roundovers actually help the sound by reducing edge diffraction. The larger the radius the more the improvement. I usually use a 3/4" radius. The larger radius also makes rolling veneer over the edges easier, which tends to be why I use it.
You can round all the edges over if you want too. That will also help control edge diffraction at the top and bottom as well as the sides. This won't work if you plan on veneering the cabinets because you can't roll veneer in more than two planes, unless you use a modified method. If you are going to use paint, a roll on finish like DuraTex, or use thick enough pieces of solid wood you and roundover all the edges if you want.