You mean use a longer aftermarket shackle to give more lift? As long as you do it within reason it should be ok. Just watch your pinion angle since it will point up more the longer the shackle you use. And make sure the shackle is sturdy enough to take the additional side loads.
I don't like shimes. Every shim I have ever installed as split in half and fallen out within weeks of installing it... Both alluminum and steel... It got easier to just replace the rear spring pads on the axle then replacing shims once a month.