Fact or misinformation?

How can it possibly make any difference whatsoever if both sides of the groove are identical (mono) or different (stereo)?

Obviously a salesperson with zero in-depth knowledge making ill-informed guesses.

Thanks for the laugh!

NO! 

@intactaudio +1

No...in fact a mono record can sound quite good played with a stereo cartridge. Especially if you engage the mono switch on preamp if equipped. You can buy an in line mono/stereo switch. I opted to go the mono cartridge route, Grado, as I have a removable headshell for easy cartridge swaps. Keep in mind though, mono cartridges/stylus should only be used on older true mono records....the later "dubbed" mono records can be played with stereo cartridges as the grooves were cut in the same manner as their counterpart stereo versions. The grooves on the older mono records were cut differently with wider grooves. 

I haven’t had a mono cartridge since the early seventies and have never had an issue.

All the best.