Am I correct in my assumption? I read it somewhere but I can't find the source anymore. Is this just baloney or is it true?
It depends on the report that you are attempting to pull and the speed at which you want to pull the report, but here is how you can tell and adjust it.
Selecting the shield will offer you a drop down to show the report with great precision or faster response. If the report is not using 100% of sessions then it will tell you what percentage of sessions are being used.