According to my understanding:
You do get "extra" -- it's based on prorating your players' IP/PA over 163, 164, 165, etc. games from what they had over 162 games.
For example, if you play 10 playoff games, a batter with 600 real life PA (per 162) will be allotted 701 total PA (regular season plus playoffs) before seeing the effects of fatigue:
600 / 162 * 172 * 1.1 (last number is the 10% bump)