IMHO, go with what the local contractor recommends, as s/he would know more than the dealer on what will support your tub....of course, on making your deck stronger, there's no such thing as OVERKILL.
You're going to want the deck to hold up to the weight of the tub, water, and people in one small area for years to come. You COULD have a pro come in and reinforce the area for you, or take their advice and do it yourself, if you're handy. Just be sure that you do it right the first time, as 7' is a LONG way for a loaded tub to fall....