Automatic Trainee Filter

No, it is not necessary to change the transmission strainer.

Some filter screens are enclosed and trap dirt that can reduce oil flow

If you've never dropped the pan and wipe clean the magnets then it would be a good idea. You can get a Fram ATF kit from your local parts store, should be about $20. It includes a new strainer and cork-rubber gasket. Works very well.

Cleanliness is important when working with transmissions. Make sure no lint or dirt that can plug valve bodies get in there. Make sure the magnets are put back to their original locations and do not block the pickup port of the strainer or interfere with anything else.

Do let us know what you find in the pan and strainer. Thanks.