I found this recent rule modification concerning chop blocks:
2-3-3 – Chop Block
It is not a chop block if the blockers opponent initiates the contact.
http://miaa.net/gen/miaa_generated_bin/documents/basic_module/2015_NCAAMIAA_Football_Rules.pdf
That said, it's a judgement call on the refs part as to who initiates contact. Unfortunately, far too many chops have been called on us when the defense has initiated contact or even
held our blocker who is charging to the next level.
In addition to the subjectivity involved, the refs are conditioned to look for it even when it's not there thanks to all the whining and belly aching of opposing coaches, fans and tv commentators. In that light, I think it would be much better if we attacked high with the blocker that is now going low. Then there is no way a poor judgement by an official can kill a play/drive that may affect an outcome.