In whichever accounting area you’re making significant assumptions or estimations in, you ought to disclose them. Assumptions and estimations are in essence subjective so disclosing them means that the readers of the report know exactly what the related numbers are based upon. Otherwise it would just be a number from the sky for them. 

When we’re talking about “significant”, what we mean is an impact that would have a significant risk of causing a material adjustment to the carrying amounts of assets and liabilities the next financial year. To give you an example if the real estate market is expected to plummet in the coming year, you ought to be very conservative in your estimations about its fair value and consider that the prices are going to fall instead of rise most probably. If the value of the asset is significant on your balance sheet, the effect of any assumptions and estimations are likely to be significant just the same.

Sources you obtain your information from that you’re basing the assumptions on is just as important since the readers are most likely very interested in knowing how reliable sources you’re using. It’s not just the reliability, but also the relevancy of the sources. For an example if you’ve got government estimations made in 2008 and 2010 which both apply to year 2011 (note that in this case the financial year is 2010), most probably the one made in 2010 is more relevant and trustworthy to be used.

All those assumptions and estimations are to be disclosed in as full detail as possible and as relevant so that the users of the reports could make objective decisions and would have all the information needed to make decisions.