Major effects on cells grown with 5% FBS supplemented media instead of 10% FBS?
If you were to run a flow-based viability assay (annexin V, live/dead fixable dyes, 7-AAD) or dual-fluorescence count (like acridine orange/propidium iodide, popular method as of late), you might find that more cells are undergoing apoptosis, or an increase in dead cells. I say this because trypan blue often overestimates viability. Your most pronounced effect should simply be a reduction in viable cells or a slower growth curve if you take those data points. Otherwise you'd need to run some test (fair example).

The problem being, these observations vary wildly based on what cell line you're using, lot-to-lot variability of the serum, and reagent handling. For example, the serum was heat-inactivated improperly, medium was degraded, bad lot of serum, and so forth. So take this as a generalization and a good reason to validate your serum lots.