[tongzilla]

It is extremely important to be exactly on plane at Impact.
It is very important to be on plane at Release.
It is important to be on plane at Start Down.
It is good to be on plane at The Top.
It is useful to be on plane during Backstroke.

There are very few pros, if any, that are perfectly on plane during from Address to The Top. Not enough 'forces' are present in the backswing to keep club on plane (that is just my opinion...not Homer's).

Edit: when I'm talking about being on plane, I include any plane shifts (angle), but not shift in plane line direction.