First I think it depends on the dog. If food is a good motivator than that is what I would start with, but some dogs enjoy a toy as a reward just as much as food, if not more.

Personally all of my dogs so far are very food motivated therefore that is what I use.

When first training a new command or for overall training like in puppies or newbies & the like I always use food for EVERY command.
Once they learn the command I start cutting back, giving them a food reward maybe every second to fourth time they obey the command. If toys are a good motivator I introduce them as a reward as well. I always try to alternate, I think it makes the dog work harder as they never know to expect as a reward. I believe in their minds they think like "hey whats mom gonna give me next, liver, chicken, my ball, what what what my oh my I must obey her to get what ever goodie she has this time".

I NEVER stop giving rewards completely, even the best trained dogs need a little reward & reminder once in a while otherwise they may stop working all together. I just cut back & alternate more.