I try to always respond. Only takes a second so why not. Sometimes people put in super low offers just to see whether the seller is responsive or not before following up with a more genuine attempt.
Or they may just be starting somewhere low which isn't necessarily a waste of time.
Say you'd be happy with selling a domain at $1,000 so you put in a target price of $3,000 - a price you'd be over the moon with. Say someone makes an offer of $100, that means you're starting out at $100 vs $3,000. If you met in the middle that would be a great outcome.
What I don't like is when the subsequent offers inch up by a tiny amount.