Well... that won't compile, as you will get a "Cannot implicitly convert type" error.
If you use integers and cast it, you won't get rounded:
int NumValue = (int) 123.6;
string StrValue = NumValue.ToString("00000");
Or, if you are using floating point then you can explicitly say what rounding you want:
double NumValue = (int) 123.6;
string StrValue = Math.Round(NumValue, 0).ToString("00000");