Not recommended.
There really isn't a reason not to.
You can still use your normal blockland executable and key even if you register it on steam.
non-steam version means alts alts alts
and in steam you can have only one key.
+no workshop
Steam version also means alts alts alts
You can still have the normal version of blockland for multiple keys.
Or even... multiple steam accounts!
And you still don't have workshop without steam.
Seriously, here's the major difference:
Registering your copy for steam
Pros:
You can have it on steam
You can still play it normally
Cons:
Not registering your copy for steam
Pros:
You can still play it normally
Cons:
You don't have it on steam
