In all likelihood Premier will show the Rangers/Killie replay on 20th Feb.
So probably best to sign up before then, as you would see rangers last match in the competition!
On the off chance they managed to beat Killie, you would still be in the free month period for the quarter final.
By delaying until the day before the quarter final doesnt really benefit, as the free month would have expired before the semi Final weekend April 2nd.
And dont forget to cancel subscription as soon as you sign up!