The rule book says that you need to decide on your tickets even before the next player takes his/her turn. How big a deal is it to allow a fellow player to decide on destination tickets while others are taking their turn?
Allowing play to continue can give the player who is choosing which tickets to pick up an advantage:
I thought I remembered seeing advice somewhere on the site that said "always let play continue if you're choosing the tickets, never let it continue if it's someone else", but I couldn't find it. While this is certainly the most "effective" way to play, I personally think it's a bit underhand.
I prefer to play as written in the rulebook, but if it's a causal game and people want to keep playing, I'll generally let it happen (no matter who is choosing the tickets). It definitely gives an advantage to the player choosing their tickets, but it's not game breaking.
Given that the ticket picking process is usually the slowest aspect of the game, and that turns are designed to be really short, when I play with my board gaming friends we play with a house rule.
The house rule is:
Even though technically picking cards of any description gives information that the ticket picker should not have, we have found that the difference that this makes is negligible and the time saved is worth the marginal advantage that we give the player.
Of course track placement is the most important information in the game and the ticket picker should, under no circumstances, ever be allowed to see track placements before making their final decision.
In a tournament setting or playing with untrustworthy players I would recommend sticking to the official rules.