I've stuck with United States singles before 1940 and plate blocks until they were larger than 4 per block. So much history is involved before 1940. After that, and especially in the last 30 years or so, so much volume came out that it quickly became unmanageable. And, the collectable varieties just weren't interesting to me. So I don't collect or sell much after 1940.
This being said, everyone has different tastes and should collect what they enjoy. Remember, stamps are generally a very poor investment vehicle. The best return on your investment is the joy of collecting.
