Problem with 100w ones is that in some cars they can overheat and blow, or melt the wiring.
Yes, good point, I checked with a few people I know and they had used 100w bulbs in the same car as mine and never had any issues (MK4 Astra) If you are unsure you can check the wiring and see what gauge it is then work out what sort of current it can handle.
This was a common "mod" on the MK2 Golfs, as the headlights were switched without a relay - leading to a really hot interior switch 

