The reason you would implement VLAN’s is to segment ARP, DNS and other traffic for different purposes over the same ethernet physical layer.
Really, it depends how many devices you have.
If you deploy 15 IP phones, have 6-8 simultaneous calls at most, and have another 20 PC’s and a handful of servers, there is no need for VLANning.
All you really need to provide priority to VoIP traffic is a QOS implementation on your router. See this other discussion for more on that aspect.
On the otherhand, 50 IP phones and 50 PC’s will definitely be stepping on each other’s toes as they all broadcast for DHCP, DNS, printers, getways, etc.
Keep in mind, if you do decide to introduce VLAN’s you will need all your major switches and routers to be VLAN aware.
Hope that helps.
I agree that you may not NEED to implement VLANs based on the smaller amount of users/hosts, but its a better practice to segregate voice and data altogether. Granted, you will need to ensure the VLANs are set-up properly on all routers/switches, but the extra time pays off in knowing that even if a PC NIC broadcast storms, it wont affect your voice traffic at all…