It depends on whether you're only following a timeline of people you've chosen to follow, or if you're picking and choosing random posts from people you don't know yet.

I do occasionally come across something. Mostly it's bots who attach their offers to the end of conversations with some minimum number of posts. Anyway, relays with antispam filters that filter out low value posts (low follower count, zero replies, etc...) probably do their job here too.