Here are some thoughts from the great personal finance book Grow Your Money!: 101 Easy Tips to Plan, Save, and Invest. They list how long you should keep tax returns as follows:
If your tax situation is very straightforward, the minimum required time to keep your income tax returns and supporting documents is three years from the return's original due date, including extensions.
If your tax situation is more complicated, save all your tax returns and documents for at least six years.
Our tax situation is fairly complicated (one reason we use a CPA to do them), and I keep our returns forever. What does "forever" mean? It means that I've never thrown one away. I have the returns as well as supporting documents in separate files in a file cabinet. I have the room, so why not keep them around?
The book then lists stuff you should hold on to indefinitely as follows:
Records supporting the cost basis of investments such as stocks, mutual funds, and collectibles.
Records supporting your home purchase and home improvements.
I keep these as well -- in the same file cabinet. :-)
How about you? Do you keep all your past tax records or do you toss (shred probably) them after a bit of time?