It’s become something of a mantra in the cinema and theater alike: Please turn off your cellphonesbefore the performance begins. Depending on where you are, some of these warnings are a bit more lighthearted while others are a little more matter-of-fact, but either way it’s practically inevitable that whatever movie, musical, or play you are seeing, you’ll be asked to turn off your cellphone beforehand. Practically. A fad that bucks this trend is becoming more widely adopted. Commonly referred to as “tweet seats,” a phenomenon encouraging the use of cellphones during a performance is spreading, to mixed reviews.