It depends a bit on what you are doing and what the conditions are like. For every day street riding, going with the manufacturer's recommendations is a good idea. For high performance, carrying loads, or extreme conditions of one kind or another (heat, cold), you may want to adjust pressures. To get a little hotter tire surface, lower pressure. To run a little cooler or carry heavier loads, raise pressures. You will want to check with somebody who knows a whole lot more than me to make sure you have the exact limits of these things, but that's the general idea.
It's a squid thing. You wouldn't understand.
Go soothingly on the grease mud, as there lurks the skid demon.