Invoice coding and discard processes

Hi all,

At the current site, our AP invoice entry users all have different ways of entering invoices, matching to PO, adding freight,  and eventually discarding any of their processing.
I am trying to write up a procedure on 'best practice' of entering and updating(discarding) invoices.
My main area of focus will be primarily around the PO matching, discarding of lines, and credit memo matching to po related invoices.

PO matching and credit memo's are not an issue for me, however, I thought I read somewhere, that the discarding of lines, should be handled in particular way: i.e. freight/misc allocations first, then Item lines. Never touching Tax lines, as they will re-calculate themselves. Is this correct?