I like re-writing GUI code in unity during runtime to save reloading while I tweak values - is this bad unity practice and is there a better way to tweak GUI values without constantly re-previewing my work.
Yeah, it’s a bad practice.
Get used to Alt+Tab between Unity and your code editor, and CRTL+P to play/stop the game. 
It’s not bad practice; it’s a Unity feature specifically that allows you to do this, although it doesn’t always work, depending on the code. However for tweaking values you’d find it more efficient to have public variables in the inspector, so you can just change those at runtime without having to recompile code.
–Eric
So long as the GUI does not rely on any variables or arrays it works fine - If you are running dual monitors this is just convenient and time saving.