Nono, some frost formation isn't a bad thing. Especially when the outside temperature is below freezing, the mere presence of frost in small to medium amounts is a good indicator that the heat exchanger is putting heat where you want it. The biggest (high SEER or HSPF) heat exchanger designs can pull full capacity up to the point where they look like they are covered in snow. So detecting the presence of frost alone is not a reliable way to control defrost.
On the other hand, detecting excessive pressure drop or delta T is much more predictable and reliable. Like any heat exchanger, there has to exist a pressure and temperature difference to force heat transfer. Deciding how high is too high on either or both parameters is key to efficiency and effective performance. Only after a sufficient amount of restriction exists for a length of time should defrost be initiated.
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