I started this a while ago, in particular when playing SoU with a fighter/thief, for more see Binky Netzen. This article talks about traps, and multi-classing and cross references a character guide walkthrough.
I have one other rogue, that doesn’t have good enough thieving skills to disable a trap, or hit points to survive it. So I wanted to know what attributes the disable traps relied on. So google is my friend, I found Neverwinter Nights Rogue Character Guide By Ross Glenn, which among other pieces of advice, states that disable traps depends on DEX, as are Reflex saves. Disable Traps is a specific skill which must be chosen, unlike in V1 rules where is is an innate class ability. The article AD+D Saving Throws also refers to thieving and traps.
It depends on the story teller, but usually monsters will get harder to deal with as levels increase and in some stories so do traps. In the early versions of the rules the level thresholds were exponential whereas with more recent rules they have become linear and common to all classes; in the early rules, up to at least 2.5, classes had different scales with the rogue being cheapest and the mage being most expensive. The upshot was that, for demi-humans, using early rules taking a dual class character meant giving a way one level. If the same points were earned, a 5 level fighter, would match a 4/4 Fighter/Thief. The fighter is giving away about 5% on it’s single class equivalent. This was ‘balanced’ by a level cap. Giving 5% away is not so bad, but in the modern rules, it’s a larger penalty, a 4/4 fighter/thief has the same points as an 8th level mage and little chance against an 8th level trap.
NWN rules also manage the XP penalty based on multi-class selections and so while human multi-class options become easier with lower penalties it needs to be managed well and it looks like balanced fighter/thieves are not such a good idea, especially in a trap heavy dungeon.