@tobee
You'd get a "no result" if you didn't do it properly. Not a negative. So that's not an issue.
Yes to the question
Not on the lateral flow tests you dont.
You can not swab at all, and do the test - it comes back negative. You can scrape your tongue, and then extract it - it comes back negative.
I believe the PCR is more sensitive to poor sampling, but the lateral flow tests are very hard to get voids on. I've seen 3 - 2 where insufficient liquid was added, and one where the line came up right in the middle of the window so we didnt know if it was a very low position control, or a very high position positive test.