From what I can tell, at least on a 10gal is that even having the rim on does little to prevent bowing. I filled two 10 gallon tanks that I had with water to test this exact thing. One tank was one that I de-rimmed, the other had the rim intact. I took the measurements of both before and after they were filled with water, so I could see how much more the de-rimmed tank bowed than the rimmed one did.
The results surprised me in that the de-rimmed tank only bowed something like 1/8 " more than my rimmed tank. Granted, there must be variation in how much a tank bows based on the strength of the glass, construction, etc, but I was surprised to see that the difference was so minimal, even though it was a small tank.