It depends whether you have other income. As Tax Champ @TomD8 said, “You'll be able to claim a credit on your PA tax return for the taxes paid to MT, so you won't be double-taxed.”
The credit only applies to your double-taxed Montana income. Additionally, PA will only credit the amount of PA due on the same income. For example, if the MT tax on your self-employment is $1,000 and the PA tax is $800, you don’t get an extra $200 ($1,000-$800) to offset PA tax from non-self-employment income. Your credit in this example is limited to $800.
Therefore, if you have other income without any PA withholding, such as from interest, dividends or capital gains, then you can pay estimated tax to avoid penalty.
