It's also important not to burn out. You are roughly infinitely more useful to your startup when you are well rested than when you are burnt out.
Wasting time helps a lot with that.
Wasting time also gives you a great opportunity to take a step back and gain some perspective on everything and see whether you're working in the right direction, otherwise how do you know you're being productive rather than just doing busywork?
If there's one thing founding a startup has taught me, it's to avoid busywork like the plague and that when you are busy busywork looks the same as being productive.
tl;dr: from the trenches being busy and being productive look the same. You need to waste time to be able to tell the difference.