only if mixed incorrectly or to thick on a panel will the filler crack unless u are shaving then it will if the panel is not fully welded or corrosion protection is put behind the weld area.
If you do a decent job of welding with minimal warpage you should be fine..I always use a skim coat of fiberal..by U-Pol..shit is great!..flexes and prevents cracking..then spread your bondo and you'll be good..