I just opt for parens every time. I really think they shouldn't ever be optional if in some cases they are mandatory. That this is a matter of personal style is somewhat frustrating.
Remembering to call a function differently depending on whether it has arguments or not has been a common source of errors for me.
It's also much easier for me to parse the difference between "foo(bar(baz))" and "foo(bar, baz)" than it is for me to parse the difference between "foo bar baz" and "foo bar, baz"
