Is it normal to put my girlfriend on top when we're kissing?

I read somewhere that couples who kiss regularly will come to share very similar bacteria in their saliva. And that when two people kiss, thanks to gravity, most of the saliva travels from the taller person's mouth to the shorter person's mouth. To solve this I like to put my girlfriend on top when we kiss (when we're in bed), and when we're out somewhere, I like to grab in behind her knees and pull her up so she's taller than me and then kiss. Sometimes I'll tickle the sensitive soft skin behind her knees too.