As for myself, I return all calls and messages. Yes, even on multiple offer properties. It takes a little time and effort, but everyone deserves an answer. I want to be a Realtor that others want to deal with, not the other agent.
When agents take a listing, they have a duty to present all offers, and respond to same. When I take a listing, it is marketed, updated, and Sold. I don't just take listings, I Sell Homes.