WHAT IT IS, IS THOSE SLIGHTLY CROOKED TEETH. LAURA Leighton, 26, hates them (“Sometimes I want to cover my mouth”). Her costar and beau. Grant Show, 33, eats them up (“They’re adorable”). And the millions of eager viewers who tune in Melrose Place each week know that when Leighton’s character, the divinely twisted Sydney, flashes a smile, something deliciously baaaaad is going to happen. (She has been a stripper and a call girl, and she ain’t done yet.) The 5’4″, 100-lb. Leighton, who was one of L.A.’s busiest waitresses before landing a two-episode Melrose gig in early 1993 (she joined the cast full-time that fall), has other assets to put her in our pantheon. But the lowa-born actress keeps the attitude homey. The sizzling mane, she says, is her natural color but sometimes “needs help in the red department.” Despite stepping out for the evening with Show from their Spanish-style L.A. home, Leighton has an ironclad rule: 8 hours of sleep. “Anything less and my entire face puffs up. If I show up on the set that way, they have to ice me down she says, with a slightly crooked smile.