I just got destruction up to around 25ish for combat skills, now I'll be doing a lot of side quests/divine/daedric before picking up the main quest again.

Is it a good idea to go ahead to start using stealth paying attention to my magic skills (restoration, destruction, alteration, some conjuration later) to keep it viable. I know stealth if not used right will be useless as you level up due to hostiles being too high level for lower sneak ability to work on them. [or is it fallout new vegas that has the mega penalties]

Sneak + illusion for a mage = badass

If you want to be a sneaky mage then the Quiet Casting perk from Illusion is a must, it oddly also works with shouts even though shouting is anything but quiet. Sneak around and put a couple perks in Sneak then you'll be invisible to most enemies, especially if you use daggers to sneak kill.

it works with shouts because shouts are spells. A spell is far from quiet normally too you know.

I don't think the level of the enemy effects the chances of detection. You just have to have high sneak skill, move quietly, stay in the dark & out of line of sight. All you need are 5 perks to get silence in the sneak tree, and you should be very hard to detect.

illusion spells however, are effected greatly by the level of the enemy your using them on.

ah, was thinking it worked like new vegas where you sneak ability was related to level as well... where higher level mobs could detect ya a lot easier if your sneak was too low