Tax documents of the time after you were in H1b. Stamping is for your H1B visa and they have time only to look for precisely H1B documents. So, if you had worked on H1B in 2010 or before then you should have the w2s and tax filing copies of those. Not to worry what you did before you were in H1B.

you can ask any questions at email id here http://nepal.usembassy.gov/visas/non-immigrant-visas.html  ( at bottom paragraph )

Remember: Embassy is there to help you and not to trouble you. If you have been maintaining lawful status and you are working in H1B lawfully, they will be more than happy to assist you in your stamping.