Based on my limited experience, the above is true, I originally bought the CE last year, go the 30 free days, and then bought the DD from D2D when they ahd it on sale and got another 30 free days, along with all the other goodies that came from it.
I could have sworn i read somewhere that they changed it so you no longer get the free time when adding more than one edition to an account.
This is true. While the bonuses stack, and I suspect you might get one free 30 day period, you no longer get 30 free days with each code. Wishstone made a post about it a while back, so I guess you could search her posts for the actual confirmation.
So there is no benefit in using multiple codes of the same type. If I were you, I would look at the various editions (Collector's, Digital Deluxe, etc.) and figure out which bonuses are worth the cost of that edition, since that's basically what you would be paying for - now that the 30 days no longer stack. They're selling for pretty cheap in various places though, so it could still be worth it, even without the 30 days. Up to you.
Stacking is still totally viable but its a bit limited now! DD versions are still brilliant though.
Boxed versions seem to no longer stack with each other at all (you only get 1x of - retail,gold,collectors etc)
Digital deluxe copys still DO stack, each adding 30 days+500c-points+whatever bonuses, as long as each DD copy is from a different retailer (e.g you can do 1xsteam,1x,d2d,1ximpulse,1xatari 1xamazon- etc)
I recently stacked another DD onto my account as of yesterday and still got everything (30days+500cpoints+a preorder item i was missing).
In total, my account has had the following added to it to date:
2x digital standard
1x Gold edition
1x Collectors edition
7x Digital Deluxe
So, the real question is, what in-game bonuses come with each edition?
I got the Steam Digital Deluxe, which has the Liberated Borg bridge officer (pre-order only), and unique-to-Steam Phaser Satellite Turret universal console. Then, for all Deluxe editions, theAutomated Defense turret Tactical console, "KHAN!" emote, "Bloodwine" emote, ToS uniforms, and NX registry prefix.
Looks like the boxed Collector's Edition has TNG and DS9 uniforms, plus the Red Matter console.