When do we toss out the negative root?

Hi everyone – I understand in most cases when we're working with quadratics and roots, etc., we consider both positive and negative roots on the GRE. However, I know there's instances we toss out the negative root. I keep getting confused when to ignore the negative root or not. Could someone please clarify for me? Is there a simple guideline or rule of thumb I can bear in mind? TIA!