Reviving old 9v - how important is gearing a the drive wheels on a steam locomotive?

After 20 years in storage I'm in the process of cleaning and sorting all my legos from about 1995-2002 for my kiddo (and for fun). I have 3 9v motors, 2 of which seem fully functional. I'm working on improving my childhood steam train moc that previously ran on mostly-decorative 30mm model team drive wheels with a 9v powered tender. So I got some 85489 wheelsin the mail yesterday, got some connecting rods on, and am playing around with new running gear. My question is, how important is it to...