Related, is people holding handheld radios (walkie talkies) sideways, popularised by 'cool guys on tv doing it'.

It's actaully very bad to do this - transmissions on handhelds are supposed to be vertically polarised (base station antennas are always vertical) and holding the radio sideways will transmit a horizontally polarized signal. This actually really matters - it can reduce by as much as 30dB the received signal strength at the other end (and -30dB is 1/1000th)