Lower RCS is never a bad thing. F-35-level stealth - my educated guess, by the way, is that it is no better in that regard than the F-117, probably not as good from sides and rear - would have baffled most detection systems from the 1980s and 1990s. Also, if you employ standoff and escort jamming (the latter from UAVs like MALD-J) you can still benefit from low target RCS.
But is it worth the money and performance trade-offs across all the missions we expect fighters to do? When the F-35 was supposed to be cheaper than an F-16 and be otherwise comparable to anything out there, you didn't have to ask that question, but now you do.
If I had to plan a tactical-range force today, I'd use conventional RCS-reduced fighters with EW and cruise missiles and a few Neuron/Taranis/X-47-type things to give the defenders the twitch. Plus a few super-OV-10-type things for CAS/armed ISR when the longest-range threat is MANPADS and I don't want to burn fighter hours.
Killface - Something about never getting a second chance to make a first impression? As you should know perfectly well, B-52s and B-1s have their own mission niches, and quite modest upgrade costs. F-22 upgrade costs are enough to replace the entire force over the decade, and if F-35 would really score 6:1 over an adversary you'd do it.