How much does underinflating motorcycle tire pressure affect grip and wear?

[Dennis74]

I just got my first bike, a used Yamaha MT-07, and I'm trying to get all the maintenance basics down. The manual says one thing for motorcycle tire pressure, but the previous owner told me he always ran a few PSI lower for better grip in the city. I'm not sure who to trust. Does running slightly under the manufacturer's recommendation really make that much of a difference in feel, or is it just asking for a pinch flat or uneven wear?