Alpine Traversing.. sleeping bags..

i think you should be ok in fair to moderate weather with a 30 degree bag if you keep an eye on the forecast. sometimes fall and the rain/snow start early. something i use to help me judge temps in the cascades is subtract 3degrees for every 1000 feet of elevation. just find a weather station near where you are going and check just before you go. where are you going?

Gonna do the Bailey.. late Aug/Sept

While unusual, I was once caught in driving rain/snow/rain for about a week in the central cascades in the first week of September. Personally I was glad for the warmer bag only because layering was a problem since a good deal of my clothes were wetted out. The bag really came to the aid of someone who terribly underestimated how bad the weather could be in September and didn't bring appropriate clothing and not enough "comfort" food.

Obviously at that time of the year (especially August) you are much more likely to run into warm days and temperate nights (especially as you ascend a bit out of the valleys where cold air tends to sink.)