What density? HVAC psychrometrics

Alright,  Ive solved numerous psychrometric problems and havent been able to pin down what air density should be used to calculate heat gains, losses, humidfication loads etc.  At times ive seen supply air densities other times ive seen return air densities and sometimes the typical standard temperature & pressure density of .075.  I know psychrometric problems arent exact i.e. have some percent error associated with the calc, but some of these answer choices on these problem sets are so close together that the density being a little off will get you  the wrong answer.  What do the experts and PEs around here say?  Help much appreciated...

This may be relevant to your question:
http:// http://engineerboards.com/topic/29306-when-to-use-psychrometric-quick-equations/

Much appreciated; thread summed it up very well.