What is the difference between ON C1=C2 AND C3=C4 and ON (C1=C2 AND C3=C4)?

In this case, there is no difference. If OR's are present in the criteria, then the parentheses can make all the difference in the world.

Shifting gears a bit and with the understanding that it's a 100% personal preference, I don't specify INNER JOIN. I normally just use JOIN to make outer joins more painfully apparent because of indentation differences. I say "normally" because, if I do work for another shop, I'll follow whatever standards they have and very few have such a standard of specifying the use of INNER JOIN instead of just JOIN.