When a creature is subjected to the Suggestion spell, at what point in time is it required for said creature to make its save? The moment the spell is cast, or the moment it receives its suggestion? Imagine a Beguiler with cloaked casting. This beguiler casts suggestion whilst completely hidden, thus getting a +1 on the save DC of the spell. The beguiler then reveals itself to its target, and utters the suggestion in question. Does the +1 to the DC still apply?
Unless a spell's description says otherwise, a spell comes into effect when its casting time is completed. The Player's Handbook on Casting Time says, "You make all pertinent decisions about a spell (range, target, area, effect, version, and so forth) when the spell comes into effect" (174), and that includes, in the case of the 3rd-level Sor/Wiz spell suggestion [ench] (PH 285), recommending a course of action. That also includes—unless the spell says otherwise—the creature that's the spell's target making its saving throw right after the caster completed the spell (and the caster knows if the target's saving throw is successful).
There's simply no time available between the casting the suggestion spell and the offering up of a recommended course of action nor between casting the suggestion spell and having the target make the saving throw. The caster can't cast the suggestion spell and, by delaying the recommendation, dictate when the target makes the saving throw.
The suggestion spell's target makes a saving throw against the spell upon the spell coming into effect. A caster that successfully affects a target with the suggestion spell that then waits 1 or more rounds until the right moment to emerge dramatically from the shadows to issue the recommendation mandated by the spell suggestion will see the suggestion spell take effect and run its course (the effect still has a 1 hour/level duration) yet see the target's behavior unchanged, the caster having offered no recommendation when the spell came into effect.