It depends upon what they find in 2010; if they don't find anything, then they most likely won't do much if anything on the other years; if they do, then you should expect that they will look for similar adjustments
in the other years.
One thing to consider; if it starts to get dicey, you may want to get a CPA involved to represent you. The audit usually goes a little differently that way, even if the CPA didn't prepare
the returns; the IRS just deals with us
differently than they do taxpayers.
If you had a CPA do the returns in the first place, it is important to involve him or her in the IRS audit; if they no longer represent you, then get another one involved.
In any event stay calm, give them whatever they want; be co-operative and if they are on site, attach a high priority to getting them what they are asking for and nothing extra. Don't volunteer any extra information, the more you say, the more they have to question.