So far if the quilt is larger than say about 50" square, I will pin the top's bottom to the bottom roller. Anything smaller I have floated and when I did the round table topper that was 60" round I floated it but I did baste around the peremeter the area of the quilt that was exposed for quilting, the rest I just let hang. I can tell you I was saying a lot of prayers when I got to the bottom of that round quilt hoping I wasn't going to end up with a big old pleat. But it laid nice and flat before I started quilting it and stayed nice and flat for me.
I do tend to baste the sides as I advance. It is rare that I don't. Sometimes I do a very loose baste like 1 stitch per inch, other times I just do my normal quilting stitch down the sides as I advance.