The stuff you have to store will always expand to fill your available storage. You won't go wrong buying as much disk space in your computer as you can afford. There's ALWAYS going to be something more you'll want to have space for.
Related to Max Out Your RAM - eventaully, if the device is upgradeable/expandable, then you may want to consider an upgrade a year or so after purchase to maximize cost/benefit. But if it's sealed (I'm looking at you, iPhone/iPod!) then max out the storage from the get-go.
This posting was inspired by the tragic tale of Duane, who just realized he should've gotten a 64gb SSD in his netbook instead of the 32gb.
This is also related to Keep Your Original Source Files in the Highest Resolution Possible.