Employee Conversations

I would like to know if anyone has any advice on conversations with employees of a target business.  I've run into a couple of situations where employees are well aware that a business is for sale.  When would people say it's appropriate to ask to speak to a few employees to get their thoughts on a business?  Pre/post-LOI? It depends? What types questions would people consider to be inappropriate or shy away from?  Any feedback around successes/ failures people have had would be appreciated.  Thank you!