The best cutoff wheels are sold by House of Balsa.
They are sold by most hobby shops and go by the name "Tuf-Grind".
They are reinforced and don't shatter like the cheap ones that Dremel sells.
You can get the 2inch and 1 inch ones. Personally I like the 2 inch as it's a lot easier to get nice straight lines.
As said before make sure you leave the corners rounded. Sharp angles cause high stress zones and will lead to cracking.
Tuf-Grind cutoff wheels