While you will probably chew just fine with first molar bite only, keep in mind that leaving your upper second molar without an antagonist (that is, a lower second molar) will most probably lead to slow eruption (downward growth) of that tooth. If that happens, be prepared for interferences in your chewing motion and loss of that tooth as well. If this is a financial decision, consider placing the first molar now and the second molar next year before the natural tooth has time to move too much. Good Luck!!