Are there dangers in using a crystal ball, mirror etc for scrying?
The old-fasioned term is "obsession" by the the spirit. In practical terms, this means that one's mind becomes unbalanced due to the contact acting through one's own subconscious mind, like an artificially occasioned neurosis.
This is most likely to occur when the magician is insufficiently self-aware, and has not made any effective effort to assimilate repressed or rejected impulses.
Are there dangers in meditation with one?
Meditation should be safer, and is in fact a good method, via free association, for making a survey of the more obscure contents of one's own psyche.
When does one cross the line between scrying and e/invoking?
When one requests the intercession or manifestation of an entity conceived of as independent from or exterior to the Self.
Is a pre/post banishing suggested?
Yes. It is a useful method by which to center one's energies and attention, and to demark the magical working from ordinary daily activity.
Any other protections/precautions needed?
An appeal to a benevolent aspect of Deity for protection and assistance might not be amiss, if one believes in such.
Any other tips/suggestions (concerning danger/safety)?
Don't overdo it. "Obsession" in the conventional sense of the word can be an indication that one is not as fully in control of the situation as one really ought to be.
If things get uncomfortably weird, cleanse, banish, and take a break from such work at least until the Moon is new, or until the next Equinox if things feel really bad.