That all depends on you, Mr. Watson
If you are going stock colors, I see no reason to paint the plastic, unless the exposed plastic (tweeter pod) is discolored. In that case, paint it.
But before you do THAT, think to yourself for a second or two. Do you want to be able to see your speakers (if you put new ones in that are all purty), or keep the stealth factory look? Either way, that factory felt better not go back in. If you want a "see=through" look, trim out all the cross-support before painting. If you want black-out, buy some new acoustic material, trim it to fit, and leave the crosshatch.
Welcome, downwardly mobile yuppies, to the dawn of the Age of Nefarious.
1993 SSEi - FrankenJinx - dead and gone.
1994 SSE (136K strong) - 180* t-stat,HD alternator, most unequipped SSE on the club, XM radio, Kenwood CD/MP3.
2006 Chevy Malibu Maxx LTZ - Custom 2.25" manifold-back exhaust w/FM-80. Probably the only Maxx with this system.