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One thing I've had a problem with is, I have a widescreen monitor, and sometimes when I write a program I find out later nothing looks right on a square monitor. So, here is a tip. I picked up an old Samsung Syncmaster 710 for $20.00 (a business upgraded their computers). Most add on video cards support multiple monitors, as mine does. When I want to see what my program looks like on a square monitor, I just right click the desktop, go to properties, then settings, and I select the square monitor and set it as Primary. I can now change the resolutions also. When I launch the program, it starts up on the square one. Just go to properties again to reset your widescreen to primary when you are ready.
No rebooting necessary.
So, if you only have a widescreen, or, the other way around, keep your eyes peeled for deals. Used flatscreens are pretty cheap.

I usually try games/apps in "Windowed Mode" to see how they look at different aspect ratios/resolution

But good tip never the less