I always check my tires before every ride, for all of my bikes. It is interesting to see the differences between them, my mountain bike tires only lost 2 pounds of pressure after sitting for 2 weeks, while my thinner tires can lose that much overnight.

Bicycle tires aren't air tight and do lose air slowly each day. I may not HAVE to check/air for every ride when I ride most days, but I do check. The act of checking does cause me to lose a little air, but to me it is worth it to know my tire pressure is where it should be before hitting the road. I am far from fast at changing tires, and it is a b*tch to even get the wheels off my Gunnar that I want to do all I can to avoid a pinch flat. At least when something happens I won't blame myself for it due to having too low pressure