Taxonomy forms are not designed to be used only some of the time (for block admins only).There is no way to guarantee the required vocabulary even exists
I see these both has non-issues. The edit form can only be seen by the author of a node or by an admin who has been explicitly granted admin rights for that node type. so it should be no problem to always show the taxo form IMO.
I'm perplexed by a need to know beforehand if a vocabulary exists. Part of the INSTALL instructions for make_block should be:
"If you want to create an RSS feed containing all your 'made_blocks", just tag them all with the same term and use that term's taxonomy feed. You may reuse an existing vocabulary or create a new one for this purpose."
So the onus for creating a vocabulary and taxo term with proper node type associations is on the Admin. This is how forum.module works, and lots of others. You can be nice to an Admin and do a check on your Admin page to make sure that at least one vocabulary exists and print a warning message if none exist.
