Hm, looks like copper doesn't flow much at all, at reasonable pressures. Neat:
https://nvlpubs.nist.gov/nistpubs/jres/47/jresv47n4p272_A1b.pdfBus bars are generally going to be half or full hard, I think? So they should be quite strong, supporting loads in the 20ksi (138MPa) range without appreciable movement. Under a bolted joint, high points in the surface may crush down, improving the connection; the joint overall shouldn't creep much, and certainly won't creep (over any meaningful time scale, by the looks of it) enough to relieve pressure on it. I'm certainly not going to say a superior type of washer is bad, not at all; rather, it looks like it merely isn't necessary.
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So, you can go for preventing rotation (lock washer) over maintaining tension, it would seem.
The corresponding figures for pure aluminum look to be significantly lower (can confirm, I've bent a thick bar of the stuff by hand, it's like hard shiny putty), even at room temperature (the one paper doesn't even show over 110°F); combined with the propensity for oxidation, an abundance of caution is well deserved with aluminum!
Speaking of hardware types, you can still go for a bronze bolt if you like, of course; this may even be required at high frequencies. Even if this doesn't affect resistance much, the matching tempco may prove helpful.
Mind, I don't know what actual standards are for using these; like I said at the top -- check your local code!
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Tim