Some of the white sparks can be interrupted, but especially with the more powerful melee enemies they cannot so you definitely want to block them. However blocking these will leave the enemy prone to exploit them.

Red sparks you always want to try to interrupt, the most important of these being the healers. Especially against a group of multiple healers sometimes you won't be able to kill a single enemy without interrupts.

Slightly off-topic; when I go spellcaster /w staff on a character, I use interrupt to burn stamina to finish an enemy. Interrupt does do damage. Often it is faster than going thru the staff attack animation, too (at least for restoration staffs).

I found it helpful to put interrupt on an easy to press key.

And be careful. some special attacks cannot be blocked. Like the NPCs with Greatswords using the uppercut power attack (windup animation, red sparks, and treating the greatsword like it's a golf club). When I try blocking it I still get sent flying and I get stunned for as long as I stunned the the guy who hit me. The only good grace in this is that the block negated most of the damage of the attack.

Its kind of funny in a way, but so very annoying when that attack still knocks me back and keeps me from capitalizing on throwing the guy off balance (because I'm still "stunned" from blocking the bloody thing).

Interrupts do more damage and often stun.