5.5.2 is still hackable, but it's just harder as it's based on a random bug (more random than 5.5.1). you could have a lot of chance and get it to work on first try, or try it 50 times (reboot, launch browser, visit an url, console freeze, try again, etc.)
when it work, be sure you use that opportunity to install haxchi into an already bought and ready to use Virtual DS console channel installed on your NAND (not USB).
5.5.0 should work, a little random, you'll need few tries
5.5.1 recommended, a little random, you'll need few tries
5.5.2 works but requires a lot of tries.
Either version you choose, you could probably stay on that version, but some homebrew might be designed for 5.5.1/2 only (not sure 5.5.0 has all the same compatibility than 5.5.1)
so, if you are on 5.5.0, you can :
update to 5.5.1 using a retail (official) game disc with 5.5.1 update on it, OR update to 5.5.2 AFTER installing haxchi.
just remember, if you update before installing haxchi, it's not bad, just harder.