When you hem a curved edge, if you serge the edge first, or even just do a straight stitch 1/4 inch from the edge... and then fold the fabric on the stitching line... fold up again and hem... For whatever reason, the stitching line makes it a breeze to get a smooth, even hem and you don't even have to measure! With a curved shape, a lot of your work is on the bias, so if you press it before you sew it tends to lay nice and smooth.
If you decided to try the oval/circle table cloth again... cataway's box pleat idea is right on. You could lay the fabric on the table before you hem it or anything and play with the how deep you'll want the fold and how much support.
A few small hook and eyes spaced evenly should hold it well, wash well, and be discreet. Avoid placing them in the center of the cloth, so you can place a centerpeice. If you place the ones on the side an inch or so over the edge, the guest will never notice them.
If do ties, you'll have to think about what the ties are going to look like when it's not pleated. (so that they aren't just hanging there) Lacing would a neat option too, then just loosen the lacing to make it wider.