It isn't strictly necessary. It depends upon the value of your FSR and other resistor in the voltage divider. With effective source resistances above 10k ohms you are going to start to see A/D conversion imprecision (according to the AVR datasheet). Whether or not that matters to you depends on your application and how much precision you really need in your FSR measurement.
The op-amp will help there if you use large value resistors (much bigger than 10k) by providing a low source resistance to the A/D converter.
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