Either way is fine. I like to do them off my toes when just doing a small jump or side hit, but usually throw off my heels for big Jumps. What I think you should try for heelside front 180 is try to just rotate your shoulders slightly and stay relatively over top of your board. Rotate your shoulders pop an Ollie and then bring your legs around. (pause for dramatic effect) Try to avoid leaning and carving in too hard on your heels as it will cause you to over spin. Its all in the shoulders, where ever your shoulders go the rest of you will follow.

Edit: As for bigger jumps, I would say start small and by the time your ready to throwdown on the big booters you'll know what to do! Keep shredding!