I'm using tinted plexi which makes the black bars not as noticeable. I guess it would depend on if your monitor had light bleed on the edges though.
When I went to my local glazier, they had one shade lighter than what I wanted in stock and for half the price of what I was expecting to pay.
So I bought the lighter one. In a dimly lit room, it's there if you're looking for it, but not distracting.
If I'd bought darker shade like planned, I think you'd have to really try to see it.
This is pic makes it looks as bad as possible, partially from using a flash and being in a brightly lit room.
You can even see the monitor frame. Notice you can't see the black bar at the top.

I'd never stretch the image. I've seen a couple vertically mounted widescreens on here with the classic games stretched and it agitates me just to know they exist.
I thought about masking off for the vertical orientation, but the Sega Naomi games are a little taller than standard 4:3 for some reason.
I'm also leaving the door open for pinball, although I don't have much desire to play pinball from the perspective of floating horizontally over the center of the table.
EDIT: In this pic, I can't even tell which way the monitor is oriented:
(my front-end layout is square and appears the same either way)

The actual appearance is somewhere in between. You won't notice unless you're looking for it, but you definitely see it if you are.