I almost always offer help if I see someone... I have even slowed to let someone catch me who looked like he was bonking (he was ok, just not as fast as my 12 mph average... but we had a nice chat anyway for the next couple of miles)

I have asked a few people while riding, and they have not needed help.

I have stopped and offered help twice while driving. The first was a woman who I saw go over the handlebars when her purse got into the front wheel. I gave her and her bike a ride home (about 5 miles) I figured her stitches would be enough pain. (I also offered to get her to somewhere for medical assistance, but she had someone at home to do that).

Another time I stopped and offered a guy a lift who was pushing his bike and had his left crank in his hand... He refused, so I assume he was close to home.

I have received a few offers of help when I didn't need any assistance... it gives a little sense of encouragement and comeraderie even if not needed.