Proper method of determining who goes first
The key here is, that players are free to choose any method they like, as long as they both agree to it, and as long as it's not disruptive to the rest of the tournament (setting up a speed-run to whoever reaches the other side of the GP venue was deemed NOT appropriate on one occasion)
Yes, some methods are easier to cheat with, or are percieved to be easier to cheat with. As a judge, if players ask, I always suggest using even / odd on rolling a die, where one player selects a die, then the other player calls even or odd. I use this method also when playing myself, although if people want to do highest, I'm fine with that as long as we're not using a single spindown. When playing people that have poor English skills, I sometimes suggest ‘highest’ since people will know what that means, but might not know ‘even/odd’.
Rock, Paper, Scissors does not just reward the ability to read your opponent, it also gives you an opportunity to study your opponent. Losing RPS to see whether your opponent tries to read your move may well be worth it when deciding during the game whether he has a counter in his hand or not. Playing a Magic tournament requires different skills, and some of those skills have nothing to do with the actual rules of the game. Trying to gain an advantage by these means is fine.