I don't expect the C's will make their FA singings (KG, Green, Bass, Terry) official for at least until the end of Summer League. There's a few reasons for this.
Obviously, the main motivation would be to stay under the luxury tax line, thus allowing them to take on an additional 25% in incoming salary in any trades. With the C's still hoping to obtain Courtney Lee through a S&T, staying under the tax line would be very helpful.
Additonally, it would behoove the C's to attempt to sign Jason Terry through a S&T as well. If the C's can avoid using the non-taxpayer MLE they would therefore be allowed to go over the apron/hard-cap. This might not matter at the moment, but could be important later in the season. If say the Celtics became interested in making a trade for a high-profile star near the trade deadline, being restricted by a hard-cap could be very prohibitive.
The Summer League comes into play, because at that point the contracts of Shawne Williams and Et'waun Moore become guaranteed. Those contracts probably only have value up until that point. Otherwise I expect the C's to cut both players if they are unable to use either to facilitate a S&T.
I expect, if the C's are unable to work out S&T's for either Terry or Lee by that time, they'll just sign their FA's officially, and then look in a different direction to fill the rest of the roster.