...by all means avoid the giants whenever possible (especially at levels below 20 or so unless you just plan on the plink and run tactic... even then, be prepared for a long run and the possibility of running out of arrows before that dude is toast).
Bears, on the other hand... ummm... I didn't have problems with bears... they seem to die more quickly than wolves in most cases (perhaps because they're larger and thereby easier targets?)... so, I can't concur with the avoid bears statement. *winks*
Also, if you're going with a sneaky-type character, might I recommend visiting Riften at your earliest convenience and getting a foot in with the Thieves Guild? There are some very cool 'quests' involved, there, and you get some really spiffy armor out of the deal (assuming you live long enough to become a member

)