To tell you the truth, I don't go very much for yaoi pairings in RK. I have never read or written a yaoi RK fic, but that doesn’t mean that I can't see the pairing. Sano and Tsukioka have and old friendship and a history which makes them really close, and of course the possibility of a relationship is there. However, I think Sano would see him more as his childhood friend and comrade than a romantic interest. For me, they are really good friends, the kind that joke and tease each other and drink together until they pass out. Nothing more.