Very.
While it's been said a lot, it really can't be overstated how important yeast health is to the final beer product. Sure, if you pitch a smackpack into some 1.060 wort, it may eventually take off and seem fine, but if you don't want to stress the yeast, and really want to do whatever you can to come out with the absolute best end-result, yeast starters should always be used, when necessary, for liquid yeast.
Some starters need to be quite large, so I doubt a 1 L flask will be enough; at least, not in all cases. You ideally need some head space, just like in your primary fermenter; when a starter is active it can achieve a good head of foam, especially when you give it a swirl.
Don't use corn sugar (dextrose); using that in a starter will greatly diminish the yeasts' ability to ferment the complex sugars that you find in wort. Definitely use DME, or saved wort from your sparge if you all-grain brew. You want a starter with a gravity of 1.030-1.040 for fresh to relatively-fresh smackpacks.
I'm sure you've heard of the yeast pitching calculator at mrmalty.com. Use it, it's a big help.