If you are not doing a starter, you should be fine just smacking the pack right from the fridge. Packs that don't swallow - provided that they are not older than say 1or 2 months - should still be viable and ferment your beer according to Wyeast. You shouldn't need to add more yeast, but having dry yeast at hand in the fridge, is a good idea, for those instances where maybe your fermentation has stalled/stopped prematurely.
Underpitching however might cause a longer lag time, a slower fermentation, longer fermentation and possibly off-flavours ( recipe, wort, OG, yeast strain dependent ). But it depends on the recipe: OG, qty of wort in the fermenter, yeast strain, yeast viability, etc.
