Since this is probably a matter of opinion, let me add one:
Always add parentheses, except for +, -, * and ^, since these are the only ones everybody is supposed to learn in school.
Even / can lead to confusion: a*b/c*d
- Usually read by programmers as (((a*b)/c)/d)
- Sometimes read by mathematicians as ((a*b)/(c*d)), with / as fraction line
Once you know multiple programming languages with different paradigms, it becomes pointless knowing all precedence rules.
Sometimes you copy some code from one language to another (e.g., prototype with Lua but finally copy some mathematical algorithms to C for performance reasons).
repeat
until (l > (a + b*c));
No bracket required, but readable in one second.
Also properly placed spaces significantly improve readability.
