As you said, this is a super common practice in home brewing. I've never tasted plastic in my wort or finished beer.
That being said I would not put boiling water in it. I once recirculated my strike water with a pump to preheat the mash tun and it formed a bubble in the the plastic. Even in that extreme case I couldn't smell or taste plastic in that batch.
As far a infection, you boil the wort so there is no need to sanitize your Mash/Lauter tun. You will not get an infection that would survive into the fermenter. (General Cleaning is all that is needed).
Last brew 9/10/16: Chinook, Centennial, Cascade IPA - 1.066 OG - 51% Pils / 46% 2 row / 3% 22.5L Honey Malt - US-05