I use starsan just because I have it around when I am moving wort to the fermenter, otherwise water. The starsan is because I am too lazy to go and get water from the tap... Even properly sanitized there are going to be more bugs in the fermenter than that little bit of water that MIGHT get sucked back. Also people have used water to top off without sanitizing the water without problems.
Don't take any more faith in anything I say than you would anyone else on the internet. If you listen to what I say, then hurt your self or break something it is your own fault, I am just expressing my opinion or experience.
THINK for your self!!