http://www.irs.gov/irm/part4/irm_04-072-008.html
"4.72.8.1.2 (09-01-2006) Formality of Valuation 1.Whether a formal valuation is required will depend on the transactions that occur with the plan and the form of the plan. •For example, the valuation in a single participant plan, a self-directed account, or frozen plan can be less formal in a year in which the plan or self-directed account receives no contribution and makes no distribution or change in investment. 2.The reasonableness of the method for valuing plan assets is based on the surrounding facts and circumstances. Except for certain employer securities held by an ESOP, there is no absolute requirement the annual valuation be based on an independent appraisal. On the other hand, it may be reasonable for an agent to request an appraisal for hard-to-value assets under certain circumstances, such as when distributions are made to plan participants."