RONALD REAGAN WORRIED about strategic missiles. His daughter Patti Davis concerns herself with ""strategic body parts.'' She covered hers recently with Kimberley (Mrs. Hugh) Hefner's dog for an upcoming anti-fur ad, the latest float in the flesh parade that is People for the Ethical Treatment of Animals' ""I'd rather go naked than wear fur'' campaign. But the Hef connection doesn't stop there. Playboy's July issue will boast an array of Patti pics guaranteed to melt Ron's jelly beans. ""I'm sure it's not something they're real crazy about,'' says Davis, 41, of her folks' reaction to her posing. (They haven't discussed it.) She doffed her duds to show off her barbell-enhanced bod and to counter a spate of ""nasty press pieces'' that pointedly referred to her as ""middle-aged'' -- a symptom, she says, of ""what we do to women in this country when they have the audacity to turn 40.'' Patti's pleased, though, with how the lens treated her. ""I said to someone at Playboy, "How come when I look in the mirror I see little pieces of cellulite that I don't see in these photographs?' I'm going to keep this issue around and look at it whenever I get down on myself.''