Its like the question on the US visa form, something like "Are you, or have you ever been part of, or associated with, a terrorist organization?"
I mean, you would have to be really dumb to check that box. It would just get you a free flight to gitmo to be held indefinitely without trial.
If you tick the box about porn you can at best be expected to be delayed in your travels, possibly have your laptop taken for investigation for several days/weeks, possibly returned completely trashed, and at worst either denied entry or paying a visit to the local cop shop for interrogation and possibly (if really nasty stuff) a stay as a guest of her majesty in prison.
So, just don't tick the bloody box. Who will ever know?
"Sir, can you show us your porn?"
"I don't have any porn on my computer"
"Sir, obstructing a customs officer in his duty is a booting* office, now show me the porn"
"But, i really don't have any"
"Sir, you are a human male, aged between 1 and dead and carrying a laptop, we know you have porn"
"Ok, you got me, its in C:\porn"
"Thank you sir, we would never have thought of looking th..... Oh my f**king god. Two people having sex missionary! Get out of our country you pervert!!!"
* - simpsons reference for those who wondered.