Like I said before yes you should worry about air sealing the ceiling, otherwise you risk rotting your roof, from the inside.
http://www.greenbuildingadvisor.com...air-leaks-your-home-your-attic-need-be-sealed
I dislike caulk for the job because the metal will expand, and over time the caulk will fail. you will also have more penetrations if you place any fixtures in the ceiling. The easy way to do it is like I said before and put up a layer of osb or plywood and primer and tape the seams and edges (edges taped to the wall top plate) with a product like Siga-Wigluv air sealing tape. Then put in 2" sleepers to attach the ceiling to, giving you a good space to run your wiring.
The approach is shown in this house:
http://www.greenbuildingadvisor.com/homes/passivhaus-budget
You could put spray foam on the ceiling but you need to cover that with a thermal/ignition barrier since you are going to be using your attic for storage. I'm fairly certain that a cover of cellulose won't cut it as a thermal/ignition barrier.
You really need the full R-value over the entire ceiling as well. Honestly I'd like to see you at something like R-60 in your ceiling, and R-40 for your walls.
You could also add the 2x6 on top of the trusses instead of at a 90º angle to them, though you will need to add blocking between them on each side at the ends. I would run which ever option you decide on by your truss manufacturer, as they change how the trusses are loaded in the storage space. The spray foam under the decking with blocking at the bay ends of the decking would be fine without running by the manufacturer.
I think the details I've laid out are just about the least expensive options to do a good job, easily at air sealing. The cheapest option by far is airtight drywall, but that takes skill and attention to detail to perform correctly, and the correct paint.
One other thing that you should consider doing now is adding in a bath fan. It seems counter intuitive to spend all this money and effort to keep air in only to expel it through a fan, but you need ventilation. Garages are full of VOCs and you need a way to get them out. If you weren't insulated they could easily escape and be driven out of your roof vents, but by insulating and doing the required air sealing you almost eliminate air exchange in the garage which will cause them to build up. A bath fan will allow you to ventilate the space reducing the VOC buildup, which will keep you healthier.
Also not every jurisdiction follows the IRC, or the most current version of the IRC. 