Well, 0.00 is not a string, so this shouldn't compile.. (unless there is a QString(double) constructor,b ut I don't think there is..)
As for comparisons, you can compare them, but when dealing with doubles there are sometimes errors in calculations when instead of 0 you can get 0.000...001 and == will return false.
A thing you can do is change a==b to abs(a-b) < epsilon where epsilon is a really small number.