The problem with Aluminium shims made from a can is the metal slowly deforms under pressure: the alloy is too soft for a permanent shim.
Mild steel is better but still relatively soft. Steel cans are made from dead mild-steel, which is a fair bit softer than mild steel. Extruded drink cans are always made from a soft metal, and although convenient, they're not ideal for shimming. I use aluminium to shim my 4-way tool post and occasionally put new in due to it flattening in use.
Dave