since you mention delay-"memory" i’m assuming you’re mainly asking about tapin~. tapin~ defaults to a buffer-size of 100ms(see help file) if no argument is given(i have a feeling tapout~ has the same default). the argument is simply used to setup the area in your RAM memory where temporary audio is stored for the delay line to read from.
as for time-sync, it IS useful. obviously you can set up a ridiculously large arbitrary maximum delay time as the argument for tapin~ and only worry about having tapout~’s timing synced, but it’s always best to streamline your patches to be efficient and one way is to make sure you only have as much as you’ll really need. in regards to ITM, in particular, tapin~’s argument cannot be time-synced(give it an argument of 4n and it will register an error) so you just need to decide ahead of time or have it calculated based on the situation by sending a new buffer-size. you probably weren’t asking about tapin~ being time-synced to ITM though, so ya, tapout~’s argument can be time-synced to ITM and it’s EXTREMELY useful.