Depends on who you ask, ask a contractor you get one answer, ask the OSHA man and you get another!
I weigh about 240 without tools. I feel more comfortable on a 250+ rated ladder. The 200 lb wont break but the "bounce" and "sway" make me uncomfortable. That's for extension ladders. Step ladders don't bother me. I usually try to buy a 300lb ladder.
If I have the option of a higher rated ladder, all else being equal, I would choose the higher rating. Not having the correct ladder rating wouldn't keep me from using it. Unless someone wanted me to do something dumb on it, then I would use it as an excuse.