If you haven't washed it, and have only dried it like you say, then there's no really need to wash/proof it unless it's dirty.
The main thing with down jackets is drying, if it's dry now and has dried without clumping then there's no problem.
If down garments aren't dried properly, the down can start to rot and begin to smell unpleasant. Additionally clumping together of down can lead to cold spots where there's no insulation left.
One tip when drying down items is to make sure you use a large enough tumble dryer, and to put it in with a couple of new tennis balls as well. The tennis balls bounce around inside and helps stop clumping from occurring. You still of course have to check on the jacket regularly, in case sleeves get snagged or anything, but it does work.
The Nikwax Down cleaner and proofer are really only to preserve and reapply the DWR (Durable Water Repellency). That's the invisible coating on the outside of the garment that makes water bead up instead of soaking in. If you splash the jacket with water and it beads up, then it's fine and needs no action (in actual fact the mere act of tumble drying alone helps the existing reactivate the DWR).
Personally, I hand wash with Granger's 30 degree cleaner, and then I spray the outside with Granger's XT Proofer spray before putting in my colleagues giant tumble dryer with tennis balls.
