Well, the only way to round the numbers to a multiple of 5 is to use the MROUND command specifically, unless you want to write your own script. So, if you can't install the analysis pack, I guess you'll just have to round them by hand.
Hey, I've been kinda working on the same thing.
To do the round to the nearest 5 you first need to make sure you have the 'Analysis Toolpack' installed. Go to -> Tools -> Add-ins -> Analysis Toolpack.
You're then going to use the MROUND command. Search for this command in help, it'll show you...