Even if I set some Workspaces to use “Render Preview” shading, save the file I’m working with as the default startup file, then don’t do anything else, then close it and open the Blender executable via a desktop shortcut, the viewport mode for each workspace always goes back to “Solid”.
Any way around this?
Not that I know of. I’ve always observed this behavior but haven’t tried to track down what the reason is for it, and if it’s intentional or not. You can save in LookDev, which for Eevee is quite similar to Rendered, but AFAIK it will always switch to Solid if you save in Rendered mode.
When I get a chance I’ll see if I can find out if there’s any reason we couldn’t start allowing this in 2.81.
Thank you - I appreciate that. It’s one of those things that isn’t mission-critical I suppose, but I was pretty curious about whether there was a reason for it. 
This is an intentional feature. Starting Blender with large scenes set to Rendered view can be very slow, especially with Cycles, so Blender automatically makes it so scenes are not opened in Rendered view.
Personally, I think this ‘feature’ could be removed, as it generally just causes confusion, and is annoying sometimes if you’ve set up your views just so.
I figured that was probably why, but I can’t think of a reason to stop people from doing it if they want to. You could always disable “[X] Load UI” when opening your .blend file and that would prevent it coming up in rendered view, so little risk of losing data because the render mode crashes or something.