I like to use an expander which is only slightly smaller than the cast bullets being used, and find it almost mandatory if bullet hardness is low, say in the 12-14 bhn range. Soft bullets aren't strong enough to expand case necks well, if the necks are thick like they are on many rifle calibers.
As for shooting oversize bullets. I reccomend it if they will chamber smoothly, because fat bullets can't tip in the chamber / throat like undersize do.
