The land portion of your total assessed value may be very relevant–it depends on your situation. With property values way down, land prices have fallen even more than the total package. Accordingly, if unimproved lot prices in your area are well under the assessor’s value of yours, then attack their valuation.
But in general, it is the value of close comps of land + improvements that counts.
Be sure to stick to the value of those comps last January, plus or minus 3 months. The assessor will ignor any other comps.