Why not take the question out of the equation all together and make a starter. That way, 1) you know your yeast are viable; 2) you give the yeast time to multiply which reduces risk of underpitching; 3) it fully wakes them up and gets them ready for the big game; 4) you can pitch at anytime after making the starter. Ideally you would wait till high krausen, but you could pitch earlier or later if needed.
If you insist on just using the smack pack... some guys have smacked em and not bothered pitching for 2 days. At the very least I would leave the smack pack in the fridge until at least the morning you plan on brewing. It only takes an hour or two for it to warm up to room temp, but up to several hours for the yeast to fully activate. I've had some smack packs not expand at all, but had ferocious fermentation just the same.