tested and doesn't seem to make a difference (or didn't to me) to the account in terms of bans, it probably makes a small difference.
Lately I have only been upgrading two of the five battlechests I run. Obsidium/Cobalt/Elementium/Saronite/Fel Iron all sell well on my server so I have been saving money and just farming them.
Yes I buy MoP for my botting accounts. There is a steady demand for whatever you are farming in Pandaria, where as for most servers the demand for lower level mats will come and go. I need to know what I make each month. It all depends if you are botting for money or just for extra goldz to your main account.