Assuming your wife earned income in relatively even amounts over the course of the entire year, the difference using the average or the spot rate should be minimal; so the average is fine. In the example where there are no bonuses and each paycheck is exactly the same amount as the other, the difference using the average or the spot is zero, and thus 100% a-okay to use average.
I usually use the average for the year, and spot rate for any substantial payments (i.e., bonus, options, or the amount due on 31 Jan 06). When I say "substantial", I mean $20,000 or more. The net tax difference (the increase or decrease to your US tax bill) is usually nil on smaller amounts.
The best place to get spot rates is
www.oanda.com. The best place to get the IRS approved annual exchange rate to four digits of accuracy is
http://www.federalreserve.gov/RELEASES/g5a/ Good luck.