I don't seal my tumbled tiles. But I don't color them in with pastels or anything. sometimes I use Sharpies for some color, but I like to leave them black or brown because then I don't get people saying they don't match their colors at home. I use Stayzon ink and that's pretty much it. Most people (when I did them for a sale) asked if they were sealed, as they didn't want them sealed. I never really thought of it when I did it, but they ended up that way.
I did make trivets last year for gifts and I used paint to decorate (with the SU! stencils) and I did bake and seal those. Just because I thought they would last longer with the hot stuff going on them. Just used a spray sealer (and just about killed us all in the house! - Winter, spray can of stuff, not a good combination!).
And with the dusty tiles, I found that if I rinsed them (and you do have to do something with the dust - wipe, rinse or something) I had to let them sit a very long time. If they have any moisture in them still, the stamping can be a problem. I just wipe mine down with a damp rag and then it doesn't take as long to dry.
And I think in other posts too people talked about red colors being problem bleeders. I do remember (one of the first tiles I did) coloring with red permanent marker and that bleeding a bit. But the other colors didn't. Weird, but maybe there is something in making red that causes problems.
Everyone has different ways to do tiles and they all seem to work. They are pretty cheap, so buy some, test them out, do what works best for you. They are so fun to do and really easy.
Candles are really easy to do too. I love the candles I have seen with the Medallion stamp. And maybe a brad in the middle of that emblem. They are so pretty. I may add some of those to my table as well.