Does it matter how I board my Titan?
Solution 1:
There is no time difference when getting in a Titan ─ if it seems to take longer for the Pilot to climb in (say entering through the top of an Atlas), then the time lost is made up when the Titan GUI comes online at a faster pace. So to answer your question directly, it does not make a difference.
However, there is currently a method of disembarking your Titan at a faster rate (around 25% faster I believe) and that is to look straight down as you hop out. This can be very useful when you are trying to take care of rodeo attacks as fast as possible before they tear your Titan apart.
Solution 2:
You can't be shot while boarding, but your titan can still take damage (unless its in its bubble shield).
Going in from different angles changes the animation and slight differences in time to board, but other than that there's no difference.