I never had a problem with fit on my first 3 boats - or so I thought. It seems to me that beginner and intermediate boats , like 19"/48cm and wider, often have a bucket that could be too wide for your butt. Likewise, advanced and elite boats <46cm/18" often have a bucket that may be too skinny.
A bucket that's too skinny is quite obvious, typically you wont fit at all, feel pressure points quickly, or develop discomfort after a short while. I think too wide is much harder to judge as a beginner though.
My first 2 boats (a SRg1 and old XT) both had buckets that were far too wide for me, but I never really thought about it because they were quite comfortable in the grand scheme. I could sit in them for hours with no issues. I only noticed how much the wide buckets were impairing my stability in rough water when I got into the Swordfish-S bucket which is aaaaaalmost too small for me, but is still ok after a couple hours and does not chafe. I noticed almost no stability difference between the XT and SF, and I think that is in large part because of the bucket fit. In fact almost immediately I felt more comfortable in sloppy conditions in the SF because the boat-butt interface was so much more connected and I have nowhere to slide around (in addition to the fact that the SF-S is just a great design IMO. I was very happy to see Fenn's take the top spots in the Molokai)
With the XT's wide and shallow bucket, sometimes the boat would just fall out from under me, but now I realize that this was because my butt would slip and slide to the low side of the bucket, throwing my center of gravity way off center and resulting in an immediate and unpredictable swim. I had never had a well fitting bucket so didnt know what i didnt know in regards to proper fit. Same with the bathtub that is the SR bucket (although its very comfortable!).
So to reinforce what kwolfe says, if you can slide around in your bucket and are in any sort of rough water, it is definitely worth your while to pad it out (or better yet, find a bucket that fits you properly without padding).
It will pay back more than you think!