When do you put "..." after a button or menu? For example, some menus say "Save as..." and some buttons say "Customize...". What is the rule for dots?

Many people believe that the rule for dots is "If it's going to display a dialog, then you need dots." This is a misapprehension.

The rules are spelled out in the Windows User Interface Design Specifications and Guidelines (what a mouthful). Scroll down to "Ellipses".

I could repeat what's written there, or I could just tell you to read it.

I'm going to tell you to read it.

Okay, maybe I'm going to repeat what's written there, but briefly:

Use an ellipsis if the command requires additional information before it can be performed. Sometimes the dialog box is the command itself, such as "About" or "Properties". Even though they display a dialog, the dialog is the result, as opposed to commands like "Print" where the dialog is collecting additional information prior to the result.