wouldn't say that you "need" to, but you should use grade 8's if possible. I once broke 3 of the 4 bolts on the front flange of my rear driveshaft in my 4runner and the half-hanging-on driveshaft just about tore thru my underbody/rear heater. Don't know if the nuts had loosened over time or what, but get some fine threaded grade 8's and be done with it.
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