When a flange is properly installed onto a cast-iron pipe with lead and oakum, the joint is solid enough that the flange does not need to be screwed down to the floor. Any other type of flange connection needs to have the flange screwed down to the floor. In your case, this might mean patching the floor first.
You have to remember that the flange and flange bolts are the only thing holding the toilet in place. Somebody trips and bangs the toilet, and it moves even a little, you can compromise the seal, and then you have a leak.
Do it right, and you will only have to do it once.