As a tax preparer, you should ask every new client to bring along a copy of last year’s return, and if available, of previous returns as well. This is especially necessary in the case of a taxpayer taking a depreciation deduction, since the depreciation rate should be consistent from year to year. Earlier returns should also give you a clue to additional deductions the taxpayer may be entitled to, or they may help you to point out possible deductions or tax saving opportunities that were missed. In the case of previous clients, you should have your own copies of previous returns to refer to.
From: Building, Marketing & Operating A Profitable Tax Practice (included in our Federal Income Tax Course)