1) Sometimes the timber can be a bit twisted.
In these circumstances I never bother to plane one side totally smooth because if you do then you might find yourself running out of 'thickness'. Quite often I will leave some areas for later but will mark them up with a bit of chalk. Note to self..restock your coloured chalk.
If there is an area that still needs planing and you're running out of 'thickness' as it were, marking the areas that are still rough often highlights the fact that you don't necessarily have to worry about that area since it is going to be removed for a tenon. That might jut save you havng to go and get some ore timber.
2) If the available timber thickness is marginal then again I won't bother to finish planing one side but just enough that it will support the thicknessing of the other side. Once that side has been thicknessed then I will return to the initial side and finish it off. Sometimes again using the chalk trick you realise that you can get in just under the wire.