Sales taxes would remain the same, in fact they would be vital to eliminating the penny.
The issue would simply boil down to the retailer rounding down or up to the nearest nickel.
The sales tax provides a nice wild-card to the equation. It would keep the retailer from being able to price items specifically looking for the round-up, because you don't know how many items a customer will buy at once and therefore you don't know how sales taxes will factor into the equation.
In fact, the additional pennies or lost pennies from the sale should be part of sales tax, so the retailer is at no risk.
I am dealing with the accumulated evidence...trust me he is quite insane...now I am sure I wil have to have another 4 hour conversation about this penny crap.....thanks Ruby