Is it good to do preventive maintenance for variable speed drives

Yes ...certainly dust/dirt being cleaned off.....muffin fans checked for stiffness...bearing failure.
Check the variable speed drive cabinet fans, filters and door seals. If the variable speed drive is water cooled...flush out the coolant system.
I would be careful tightening the nuts on the IGBT's...and if one is loose check that the spring washer has not cracked.
Screw down all the control terminal connections and as a final check run a screw driver over the wires to make sure one does not pop out.
Bus bar connections checked ......I used to put a bit of red nail varnish on nuts so if the nut moved in the future, you would pick it up....
As far as taking things apart....especially if the variable frequency drive is old or has been aged due to heat...can be an embarrassing moment when you put it back together....
If the variable speed drive is in a moist environment...i.e. in the hull of a vessel. Make sure it heats up for 24 hours...there is a problem with high moisture content in the air. It can produce a very expensive bang....I think there is moisture in the recessed gate terminal of the IGBT modules this causes a delay in the turn off...resulting in a shoot through...resulting in a bright red face.

If the variable speed drive has run 24/7 ...then a switch on and off test would be in order...but not if it is involved in critical operations. Things just don't turn back on??...I've not investigated why this is...it used to happen to PC's used in factory applications and touch screens....exactly why...who knows?
The use of thermal imaging equipment is a fantastic tool....but they used to be expensive bits of kit.
Have a look at the bank capacitors ...check for physical damage and check that they are not swollen. If you find one that is swollen, it might be prudent to replace all of them.
A quick check of the output voltage and current ....must be balanced, could be in order....although most variable speed drives now check the output current as part of the control loop.
Make sure no tools are left in the panel....
A nice tick sheet inside the panel to show what was tested, when and by who would show some planned maintenance structure.

Personally I would not want to do preventive maintenance on plant equipment without spare parts, backed up software and the correct tools to rebuild the system (software and hardware).
Clean the variable frequency drive heatsink and check or replace the fans. I like new fans every five years. I also double check the drives installation. This is a good opportunity to make corrections to the install like conduits or cable routing. Check the connections but don't give the terminals a crank. They should be re-tightened after a couple of weeks after the commissioning. After that giving them a crank every preventive maintenance is asking for a broken terminal. At our facility we have drive that have run for 20 years with preventive maintenance performed every six months. more...