I have found that ideal register locations are rarely obtained in most home construction projects due to Architectural constraints. Register manufactureers publish data that is used to match a rooms load requirements,with a specific throw and velocity. Rarely are these able to be achieved because of the way the duct approaches the register.I have found that ceiling registers sre the easiest to properly install but then the branch duct is usually exposed to more extreme temperatures. .Sidewall registers are not exposed to temperature extremes and can easily be put in conditioned space but the approach duct is not ideally configuered for the air to flow through the register, usually due to some type of architectural constraint( small soffits).

Agreed. Usually they just slap a register (actually 'diffuser') in the side of the duct, and rarely even reduce the duct size, nor install a device to direct air through the diffuser.