Headphones plugged into amp out???

[rolloman]

Does the impedence of the headphones I plug into my practice amps output jack have to match what the Ohm value of the speaker in the amp is?

[R Whittington]

It's not crucial that they perfectly match. The issue you'll run into (in the case of the headphones being low impedance) if they're not at least close would be a lack of volume in the headphones.