@mmat1 - There is no turn off option. However it only triggers when you try to do something where the file *must* be in a valid state for Sigil to continue.
As for the line numbers, 99 times out of 100 they are completely useless. Blame the crappy Xerces library that Sigil uses. It can only report what the *parent* element is that has the error. So if say you were missing a closing </p> tag, the parent is going to be the closing </body> tag. You could have an error on the first of 1,000 paragraph tags, and it can still only tell you that the error happened when it reaches the closing </body> and that is the line number it gives back...
So rather than jumping to a point that in all likelihood is not where the actual error is, we keep the cursor where you were doing your edit, which is most likely where you screwed up and made your html invalid.