By default it would be better to separate servers by technologies because this way you aren't excluding folders/files on servers that shouldn't be excluded.
For example i've seen some customers that needed to exclude all .log files on specific servers for some reasons but they didn't want (of course) to exclude all .log files on all kind of servers.
I'm assuming you're referring to VirusScan policies. Laszlo is correct that seperate policies would be best. It's fine to create a general server/workstation policy so systems have a policy when they first check into ePO, but when you start dealing with SQL, AD, Payroll servers you'll want to create individual policies for best performance/security. Also the exclusion interface is not the easiest to work in so I highly suggest taking your time in the beginning to save you time later down the road.