Rounding to the nearest multiple (ie, 2:{4, 6, 8...} or 5:{10, 15, 20...}) of a number is a pretty simple procedure. My solution is to use a similar process to finding the absolute value of a number, by applying a process, then applying the inverse.
To find the nearest multiple of 5, take a number, divide by 5, round then multiply by 5.
var myVariable = 8.6;
myVariable = Math.round(myVariable/5) * 5;
trace("Rounded to: "+ myVariable); // Outputs "Rounded to: 10"