I don't think they could have done that in Lewis' case, because they gave him a max contract. That means, they gave him the maximum allowable starting salary, and the maximum allowable raises every year. It wouldn't have been possible for them to give him a higher starting salary than they did.
In terms of contracts for less than a max salary, front-loading is something that should be considered; I know the Bulls have done this on several of their contracts (Wallace, Hinrich). I think Camby's last year salary also decreases on his current deal.
I think one reason that more contracts aren't front-loaded is that you need to be able to fit the first year salary under your salary cap. So, it's easier to pay somebody $10 million in their first year, and move up to $15 million by their fifth year, than it is to do that in the inverse; a team might have $10 million in cap room, but not $15 million.
As an aside, from what I understand annual decreases in a contract are limited to a maximum of 8% from the prior year's salary under the contract. This does not apply to extensions, however (which is why KG's salary can decrease next season by such a substantial amount.)