How about this -- core environment should be setup in user's non-login shell, especially if this is a daemon-like user (as the case of postgres -- it owns a service).
However, each instance of a service (e.g. eache separate postmaster process or aolserver proces -- whatever) should include 'tweaking' of environment as and when required.
For example, you may want to run 2 Pg's, listening on different ports (could be set as a env. variable and used in postmaster.conf), using different PG_DATA. Yet they both may be the same Pg version, hence use the same libe (PG_LIB) and binaries (PG_BIN or whatever), ahve access to the same system binaries (PATH), etc.
