A description replete with adverts of how to do this can be found on lifehacker here. It suggests installing a little tool which not everyone will have, so to do this on most any Windows workstation in Powershell just follow these steps in a powershell console while logged on in your desired Windows profile (minecraft saves game files per user.)
Run Powershell as an administrator (reuqired to create a symbolic link)
cd .minecraft # note the prceeding dot here on the foldername <-- span="span">-->
mkdir $env:userprofile\DropBox\Minecraft -force # Assuming your dropbox folder is still the default.
cmd /C "mklink /D $env:userprofile\DropBox\Minecraft\MySaves" $pwd
# running this in cmd is the easiest way to path to mklink from powershell that I know
symbolic link created for C:\Users\conradb\DropBox\Minecraft\MySaves <<===>> C:\Users\conradb\AppData\Roaming\.minecraft
Dropbox will start adding all your saves into the cloud, just watch your space consumption! Do this on each PC, and viola.
Testing Thinking, Anyone?
10 minutes ago