Should I see this warning when I have ISNULL check at SUM function and changing NULLs to 0?
If this setting is ON, then yes:
It's a harmless warning for the circumstance you're seeing, and since this setting has other effects it's better to leave it on.
When using SUM(), you don't need to change NULLs to 0 as they will simply be ignored.
The fact you're trying to replace them with 0 and are still getting the warning tells me something is being overlooked, but rather than try to determine what that could be... I recommend just removing the ISNULL(), as it is unnecessary.