Well, the "direction of travel" icon is not really useful on the seat reservation page, especially as the direction of travel may frequently be subject to change without notice (for example, the 9:20 train from Frankfurt Airport to Stuttgart usually travels "in reverse order" - don't ask me..).

However, on the main route from Berlin to Munich (via Stuttgart) which you are likely to take, second class coaches usually lead (low car numbers leading) between Berlin and Frankfurt. This means the side with the compartments (except the coach with the infant's compartment) will be facing the *reverse* direction of travel (reserve a seat facing away from the compartments if you want to sit facing the direction of travel). But never mind, unless you are travelling at peak times you'll probably be able to change your seat to a front-facing seat even if you have reserved a backwards-facing seat.

Here's a short overview over the main routes that I have experience with:

Berlin-Munich (via Stuttgart), ICE class 401: second class (high car numbers) leading (reverse between Frankfurt and Stuttgart)
Munich-Berlin (via Stuttgart), ICE class 401: first class (low car numbers) leading (reverse between Stuttgart and Frankfurt)
--
Hamburg-Stuttgart, ICE class 401: first class (high car numbers) leading (except Frankfurt Hbf to Frankfurt Airport)
Stuttgart-Hamburg, ICE class 401: second class (low car numbers) leading (except Frankfurt Hbf to Frankfurt Airport)
--
Basel/Munich-Dortmund, ICE class 403: first class (car 29/39) leading (reverse before Stuttgart)
Dortmund-Basel/Munich, ICE class 403: second class (car 21/31) leading (reverse after Stuttgart)
--
Cologne-Amsterdam, ICE class 406: first class (car 29) leading
Amsterdam-Cologne, ICE class 406: second class (car 21) leading
--
Bruxelles-Frankfurt, ICE class 406: first class (car 29) leading
Frankfurt-Bruxelles, ICE class 406: second class (car 21) leading
--
Cologne-Berlin, ICE class 402: second class (car 31) leading
Berlin-Cologne, ICE class 402: first class (car 37) leading