Manual J or not to manual J?
"Most" contractors take the opinion that if what was there before did fine then it is a waste of their time to do another load calc, this only works in their perfect world! Unfortunately homes change over the course of time. People install more efficient windows, add insulation, replace siding and have the house wrapped while doing so. If any of the above changes then another manual J must be done.
As the owner of the home for less than 2 years you probably don't know for sure what changes the structure has been through. To me it would be common sense to re-calculate.
I also understand why most contractors don't want to do a calculation on every job as it can be time consuming. When you are trying to get a salesperson to 4-6 homes in an 8 hour day a load calc on any of them becomes an impossibility.
Look for a contractor that isn't in as much of a hurry. While you're at it visit his shop and take a peek around. If possible take a good look at the outside of any of his trucks that might be around, while you're at it take a peek inside one of them. Is his shop clean and orderly? Are the outside of his trucks clean? Are the inside of his trucks orderly and free of trash? The back of the truck might have the whole floorboard covered with toolboxes and large parts in boxes but it should have a rhyme to it.
If the shop or the trucks are dirty and or disorderly then chances are the work he does is the same way. At the least he is less likely to be one to take the time to do a proper load calc when he also is most likely not to allow his employees to take the hour or so a week to keep trucks clean and orderly.
When you find the guy that will do it right he won't have the lowest price! His employees might drive around in rusted out vans but every spec of that rust will be clean. His shop area will be clean and orderly. The most important thing though, he won't be in a hurry!