- Is it a completed action? Do you want to focus on the completion of the action? is that important to mention?
"I killed him." period. It's over. -> use Aspect suffix
"I ate the pizza." It's gone now. -> use Aspect suffix

If you are still not sure, of course you can ask someone, but if unsure, it's better to avoid it. That's never wrong.

This is not true. By omitting an aspect suffix, you are explicitly saying that the action is not complete and not continuous. You cannot do this for an action you intend to mean is complete or continuous. Aspect suffixes are not used for optional emphasis.

pItSa' vISop CANNOT be used for I ate the pizza; it's gone now. It can be used to mean I (like to, tend to, often do, do in fact) eat pizza, but it EXPLICITLY means that pizza-eating is not completed or continuous.

-- 
SuStel
http://trimboli.name