Cytokines and growth factors degrade over time, even when you store your medium at 4dC, and deteriorate even faster at 37dC (whether in culture or warming in a water bath). However, there is usually enough of an excess. For really finicky cells that require large amts of FBS or exogenous growth factors, you can always aliquot the amt you need (e.g 30 or 40 ml) and warm just that.
Hope this helps...
-sam