rocky,
there are 2 main issues of concern when dealing with brass.
1) It is easier to spot problems (such as hairline cracks) on nice clean brass.
2) Fouled brass tends to foul dies
When it comes to mechanical issues (like press-forming brass) - clean is always better. It's possible to become too anal about cleaning brass - but it's hard to imagine how.
Oh....... In answer to the question "how often do I tumble".........
WHENEVER I FALL DOWN !! <img src=icon_smile_big.gif border=0 align=middle> <img src=icon_smile_big.gif border=0 align=middle>
"Every moving thing that liveth, I give unto you as meat" (Gen 9:3)
Trust God..........but keep your powder dry!