Which Way?
 

I'm planning a trip for October. I want to visit DC, NYC, Chicago and San Francisco.
Should I start in the east or the west?
If I start in SF, my trip would have to start in Sept. as I need to be in DC at the beginning of Oct. East start: I would start in DC and work my way west during Oct.
I haven't talked to my (possible) travelling partner about this yet; I just wanted to ask the opinion of some knowledgeable people first.
Is September more likely to be in the 'tourist' season there? I believe October is not really in the peak travel period...

The tourist season is over by September, so you can go either way. However, you might want to consider the earlier trip, as the weather will be milder.

I might consider doing NYC in late Sept, then DC when you have to be there, Chicago, SF. I say this because traveling west you end up with longer days due to the time change. The flight from DC to Chicago is probably ~3 hours, so you'll land 2 hours after you take off, but going the other way you'll land 4 hours after takeoff. From Chicago to SF is more like 5 hours, with a two-hour time change. I know it's only a matter of hours, but I just can't stand losing that time during the beginning of a vacation - I'd rather have the lost time come on the trip home at the end. :)

Skip all the cities you mentioned. Travel is about nature. And by all means take my advice no matter how screwball, wacko, or insane it may sound.

From other posts, I learned that you're coming from Australia (click on your name to find all of your old posts).

I'd suggest trying to break up your tourist time vs. your working time. You'll enjoy each city more and have something to look forward to at the end of your labors. Fly through San Francisco and onto Chicago in September for best weather. Fly to DC in early October for work. Take the train to NYC for a long weekend. Return via Chicago to San Francisco and home. This should give you the option of buying round trip tickets, which are generally cheaper. If you go to the major airline sites and register, you can get notices of cheap fares. BTW, Chicago has a nice downtown youth hostel for less than $30 a day. See
http://www.youth-hostels-in.com/hostels.htm