Depends; casting Silent spells? Verbal components pose a bit of a problem for stealth - what with the speaking in a strong voice; sure, the illusion says exactly the same... but one voice comes from a spot the critter can see, and an identicle voice comes from a spot he can't... knowing deceptive magics exist, what's a likely choice for an intelligent opponent? Casting spells without an obvious effect? Spells such as Fireball, Lightning Bolt, Enervation, and Chain Lighting (to name a few) specify that there is a visual effect emanating from you (bead flys from you, bolt starts at your fingertips, black ray, stroke from fingertips, and the like) which isn't going to be hidden, and anything with eyes to see can tell what square it comes from.
Mind you, you could invisibly move after casting a spell, and no longer be where your location was given away.... or you could prepare Silenced spells that don't have such location-oriented displays.... in which case, it would work quite well.
Edit: a little late, but there is one question I can answer:
Quote:
Verbal (V): A verbal component is a spoken incantation. To provide a verbal component, you must be able to speak in a strong voice. A silence spell or a gag spoils the incantation (and thus the spell). A spellcaster who has been deafened has a 20% chance to spoil any spell with a verbal component that he or she tries to cast.
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emphasis added)
To cast a spell with verbal components, you must speak in a strong voice; the base DC to hear people speaking (a good start for you casting a Verbal component) is 0 (distance modifiers apply...); the DC to pinpoint the square is 20 over that. So DC 20 for the demon you are standing right next to when you cast the spell to pinpoint your location. DC 30 if you are 100 feet away; 40 if 200; 50 if 300; if you are at the limits of the range of your Fireball (at caster level 15th, that's 1000 feet), I'm unlikely to even bother rolling.