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You only really need QoS when there's enough congestion that it's adverse to the service requirements of the traffic.
Many times LANs have sufficient network resources, they often don't need QoS, but QoS might be applied just-in-case (especially when supporting something like VoIP).
As maintaining even a simple QoS policy on many devices with many ports (without automation) can be maintenance intensive, AutoQoS might be used as the base policy (it can be selectively overridden, if needed).
Current generation AutoQoS isn't too bad (i.e. MediaNet, V4 version), but to leverage it, apps need to mark traffic to conform with AutoQoS expectations.
When supporting any QoS policy, ideally you want to be able to monitor how well it's doing. Sometimes QoS policies do require adjustment for your traffic. Sometimes more network resources are required.
Although QoS can be very powerful, maintaining it can be expensive and so, especially on LAN, other solutions might actually be more cost effective. For example, instead of constantly tuning QoS to manage a congested LAN link, it might be less expensive to increase bandwidth via a "faster" link or Etherchanneling. Or, instead of managing a congested link with QoS, perhaps traffic might be placed on different links (e.g. VoIP VLANs have their own dedicated interswitch link).
