Title covers it mostly. The version number is shown inside blender in the lower-right corner. From the hashes I can’t really tell what is new and what is not.
It would be nice if this version number could also show up on the website, so I can see if I should download a new version - and also be in the filename of the archive (preferably before the hash, so it sorts nicely in the file explorer).
Edit: This belongs to right-click select I guess, I’m sorry.
I don’t think the sub-version number is particularly useful as it doesn’t increment every time there’s a new build. I believe it’s really the .blend file content version or something like that and so it only changes when new or changed information is stored in the .blend files produced by that version.
So currently you really have to look at the timestamp of the build or the associated commit hash to tell two versions apart.
I agree it would be nice if there was a true build number incremented in the nightly build process, but the short answer is yes, you should download a new version because the experimental builds generally change every day 
You can look at the diffusion commit history to see what’s new in the last few days:
https://developer.blender.org/diffusion/B/history/blender2.7/
https://developer.blender.org/diffusion/B/history/master/
And given the hex commit hash on the download page you can look for it in the history to see exactly what’s in last night’s build (the commit with that hash and everything earlier).
For the most fun, you can setup your own build environment (not hard to do) and then update and build Blender whenever you want to have the up-to-the-minute latest changes.
Wasn’t aware about that link, thank you! It’s like the never-ending release notes story
Tbh, I feel a bit tired about downloading every day, now this is a good tool to make an educated guess whether to update now or maybe next day.