If you have IMS and/or ETag and the feed hasn't changed and you do a GET with IMS / ETag then you won't be sent a body unless it has changed, and which point you actually want it? Or am I still being dense?
The overheads per operation are significant, so I prefer just to get conditional GETs and no HEADs at all as a baseline. https://www.earth.org.uk/RSS-efficiency.html