It depends on how much of a stash you like to keep. Try to think of the endgame rather than what's annoying now. If you think you'll always be happy with 200 and never need to go higher, then you can certainly start selling. I'm right about where you are (one step behind, actually) with 190 and nearly enough items to get to 200. The number of required items to upgrade is annoying, no doubt. But even right after an upgrade, when I have no upgrade parts in my inventory, I still end up getting caught up in upgrades, shipments, and Vu destruction, and end up running out of space occasionally. For that reason, I'll continue to upgrade several more steps.
