Proxy files are great for most of my editing work-flow, but usually towards the end of the work-flow is where I do colour-grading, sharpening, and other "touch up" work that's best done with original files, not proxy files.

What's the best/quickest way to switch back and forth between proxy files and original files?

Thanks...

Does it work when you go to Project/Project Settings and in the Settings tab deselect Proxy Clips?

Assuming that this works (I have never used proxy clips in projects yet) I wouldn't know a quicker method.

So far, it seems like the best way for me is to select one or more clips in the project-tree, and then toggle proxy on/off from there.

Under Project>Project Settings>Project Files you will find the option to Delete Proxies. (for the whole project)
This probably isn't helpful in most situations, as you are likely to go back to editing and will want the proxies again - particularly if they are now weighed down by extra color effects etc. You will need to re-create your proxy files afterwards, which takes a while, but at least they will show the updated results of the effects you applied.
I just thought I would mention it in case you haven't spotted it yet.