You never have trip guarantee for any MU assignment until you block out.
Section 4.O. Minimum Guarantees and Other Pay Provisions - Base Standby Pay
A pilot on base airport or base hotel standby shall be guaranteed minimum standby pay of an R-day value, except as provided in Section 4.O.2. (below).
If a pilot eligible for substitution is assigned a base stand-by, (as provided in Section 25.H.5.d.), and does not block-out on a trip, he shall be credited with 6 CH towards his substitution guarantee. If he blocks out on a trip, he shall be compensated the greater of trip guarantee for such trip or substitution availability credit.
If a pilot on base airport standby blocks out on a trip, he shall earn the trip guarantee for that trip, with the calculation of MPDP and duty rig beginning at showtime of the standby period.
If a pilot on base hotel standby blocks out on a trip, he shall earn the trip guarantee for that trip.