Absolutely, a dry/compression/breathable bag combination no matter if you have down or synthetic sleeping bag. I use Sea to Summit bag for my down bag which is stored in a waterproof pannier. Having a dry sleeping bag is very important, so some waterproof redundancy make sense to me. The size depends on where it is going to be stored. You my need a relatively tall/skinny bag or a short/fat bag to fit properly inside your particular pannier and around its other contents. There are several knockoffs of the Sea to Summit eVent compression bags on the market that are worth looking at.

My down jacket goes into a small dry bag, as do my meds and supplements. I have a solar charger and the vulnerable electronics go into a pillow style dry bag underneath the solar array when charging. My handlebar bag is water proof and perhaps my most important dry bag in that it contains my passport, camera, money, chocolate bar, notebook and all the things that have to be kept dry and in my possession at all times on and off the bike no matter the weather.

I keep my tent in a DWR breathable bag I made just for my rear rack. I want my tent, which is often packed in some state of dampness to dry out as much as possible during the day. Whatever bag you use for your tent if it is exposed to the Sun all day long it needs to provide excellent UV protection to the vulnerable nylon tent fabric. If you are going to pack your tent inside a pannier, especially a waterproof one then I strongly recommend a waterproof dry bag with a roll down seal to keep everything else in the pannier from getting damp.

When my wife tours with me see uses Ortlieb Front Roller Classic panniers on the rear rack and her eVent Sleeping bag stuff sack is more tall and skinny than what I use for my only slightly larger sleeping bag.