This is certainly a personal decision, and there is no right or wrong answer. Each painter has to decide for him/her -self. However, let me offer a dissenting view.
Varnish definitely affects the look of the final product, and in my opinion reduces the highs and lows resulting in a generally "flat" appearance which takes the "pop" out of the painting. If you are willing to accept that as a trade-off for the protection, I can't argue with you. I don't accept that. I'd rather be a little more careful with the finished flats.
In some 20+ years of painting, transporting and showing flats without varnish I have yet to damage one. I paint with oils, so I can't speak for acrylics, but I assume they are the same or nearly the same durability as oils if fully dried. These pieces are very durable even if dropped on their face.
I do not use glass on my frames. I do try to use a deeper frame, but many of my frames are fairly shallow, so the flat is "out there" and unprotected. For normal display and handling, I have yet to experience even a nick. When traveling, I put each frame in plastic bag, having first covered the flat itself with a piece of tissue to protect against rubbing in transit. Never a problem. Dusting? I used a soft paintbrush.
My point is - with a little care, there is very little worry of damage; so why diminish the beauty of your piece by varnishing it?