This is a good point at which to know your rig. K2UE, I would love to know your definition of a modern exciter. While I realize that my FT-897D is starting to get a bit long in tooth, it isn't that old at about eight years. The FT-897D Technical Supplement states: "the transmit signal is passed through a low-pass filter (1.8-29.7MHz)..."
Such a low-pass filter will be unable to meet FCC requirements for IMD, when fed through the front-end of a tube-type amplifier. I recommend reviewing W8JI's article, found here: http://w8ji.com/tuned_input_circuit.htm
. Rigs from 40 years ago, running tube finals, had Pi-filters using series inductance on every band. We didn't have to worry about second- and third-order harmonics, because the transmitters dealt with each of the bands independently. I will continue to build my tuned-input board and recommend that other hams with older amplifiers follow-suit.
Another article you may enjoy is by W3EUT: http://www.rfampguy.com/Pages/TUNEdinput.html
By modern rigs I meant exciters that have an LPF for every band that cuts of below the 2nd harmonic on that band. Tube exciters with Pi-Network outputs would have been no better than that.
But your point on the need to conduct cathode current harmonics to ground is well taken, since if they are unterminated they drive the cathode out of its desired linear operation, and provide the mixing vehicle for IM generation. An antenna tuner, unless it provides a capacitor output filter, and is VERY close to the PA is not much of a substitute for a pi-network at the cathode.