by concepts, you must place only one or two switches in vtp server mode, and the others in clients or transparent, depending in the needs or the desing of your network.
Remember that only the switches who are servers are able to create the vlans, that are propagated across all the vtp domain to the clients switches by the vtp protocol.
so you must setup vtp pruning in the server one(s) to prune or prevents the propagation of the vlan info across the trunk ports to the other switches where there are no host o ports associated to the vlans that exist in only specific switches.
What is the worst that could happen if I enable VTP pruning in a network or 20 switches, including 4507s, 2950s, and 5500s?
I'm not sure about pruning, but i do know that is is possible for a client to override the server and wipe out all vlan data. VTP keeps a count of the configuration changes that is reset to 0 when vtp domain changes.
if you configure a client using the correct vtp domain and make enough changes to increment the change counter to a value higher than the vtp server and then plug it into the network, the vtp server will think it is behind and try to play catch up and then send those changes to all other clients, destroying or changing your vlans.
I've seen it happen on a fairly large network and it took a few days for them to fully recover.
Always reset the domain before plugging it in to reset the counter to zero.