Do I really need to implement QoS?

It is usually recommended, but whether or not it is needed depends on the nature of your network and how likely you are to saturate bandwidth on any one of your links.

For example, a switched network environment typically means that QoS implementation on the LAN would be needed for large campus environments, but would not necessarily be needed for smaller offices.

Implement QoS on the egress WAN connection (hand-off) if upload bandwidth has the likelihood of being saturated. If not, avoid QoS as it could cause other traffic that is not voice to bottleneck.