The fact that you were able to edit tax tables indicates that the post-dated invoices in question have already been dealt with by Day End. You should be fine.

The tax reports may have 'merged' information from tax tables and Day End, depending on the timing of purging the reports before and when the tax table edits were made. You may want to preview one or both of these reports to ensure what information gets printed (ie any invoices prior to July 1, or not).