Yes, the cranking power does drop off, and if you are already experiencing problems then change it out -- especially with winter approaching. My mom replacing the original battery in her 03' Taurus last Nov. and claims that she was having trouble starting her vehicle for a month. Eventhough she never needed a jump, it took longer and longer to crank, and then one day she left work and barely got the vehicle started. I had the original battery in my 08' Ranger that on Valentine's Day last year refused to start after I did some running around after work. I happened to be in the parking lot at Wal-Mart, and had to call roadside assistance. Not only was the battery under 5 years old, but the dealer had the vehicle in Dec. for warranty work and inspection. They specifically said everything was fine, including the battery. In April, I had an oil change in my Ford Edge, and they tested the battery (as part of the 22 point service) and said the strength was showing 75%. When I went to another quick-change 4,000 miles later in July, the battery strength was at 55%. I went to a national chain three days later, and they confirmed the low charge, and although no one can predict when the battery will give out, 55% was below my comfort zone. I didn't want the wife to get stranded somewhere, so I suggest with winter approaching to have it changed.
