The ideas of a plagiarism check is to check if you have plagiarised. If you can access a similarity score by using a plagiarism checker then use a free one. However, the idea of doing some work is not to copy and repeatedly reword until you have found some way of confounding the plagiarism checker. If you genuinely do the work after formulating your own ideas following research then you will pass whatever check is put in place.
This obvs will depend to some extent on the work set.
Should the work be "solve this sum. 2+2" It might be very difficult to generate a non-plagiarised answer so far as the checker is concerned, however, I just did that all in my head. Did I copy? Is a memory actually a copy. Good question.