Also,

Vfe
Vne
Va
Vlo (amphibians)

Note the colors (ASI) and placards on the aircraft.

are in indicated airspeeds. Indicating to slow may bend something. It has happened.

Traffic patterns are in indicated altitude. Just a few of my personal reasons to keep it straight and accurate.

You most likely have a leak in your pitot/static system. Airspeed under-reading is an indication of a pitot line leak. Have your pitot / static system tested by an avionics tech.

If you are a do-it yourselfer you can build this rig to test your pitot/static yourself.

http://www.iflyez.com/manometer.shtml

A lot depends on how you fly and how much money you have. I didn't have the money and I didn't use either airspeed indicator or altimeter so I never fixed one in fifty years.

As Cary and others say, having things working may give us peace of mind. I was a CFII with many military hours IMC in helicopters, but I would land my personal "pile" in a field or on a road before going IMC.

Also peace of mind can be dangerous for pilots who have too much confidence in their panel and too little confidence in their IMC skills.

Altimeter - shows low all the time (you set the barometric pressure and you are reading low with the airplane sitting on the ramp)? Or the altitude drops when you start moving?

First case would suggest you need to adjust the Kolsman window thing. Not hard. But it would have nothing to do with the static system.
Second suggests a problem with your static system - but usually you read higher when it leaks.

Airspeed. Could be an error in the meter. Could be a leak in the pitot system. Not likely to be a leak in the static system - that usually results in a higher airspeed. Or, could be your airplane just isn't as fast as one would hope because of rigging, fairings, whatever. Or, it could be that the instructors airplane had a broken static line and was reading 5 mph fast.

FWIW: My homebuilt has the static just open to the cabin - I can get a 5 mph change in IAS by opening and closing vents when at cruise. The difference would be smaller at lower speeds. It's hard to see the difference in indicated altitude. And, yes, that's on my list of things to do something about.

TCJ ans Lesuther,

Thanks for the info on the Kollsman adjustment. Adjusted it over the weekend (as simple as you said) and looks like it's fixed. Need to hit some strips at different altitudes and see. On to the airspeed thing now.

Thanks again.

Frank