usually I leave for the Asylum after doing Nalia's quest, clearing out the circus tent and doing the jobs for Bloodscalp. that gives you enough money for the trip and you still have some left to buy a few goodies, plus you pick up some nice equipment on the way (most importantly, some +3 weapons like FoA or the Frostreaver).
once I skipped Nalia's quest and then had to fight the Balor in the Underdark without a single weapon that could hit him
. I also didn't have the Lower Resistance spell so it proved an almost impossible fight. finally after several reloads I managed to hit him with a Magic Resistance spell and Disintegrate him.
other than the Balor, the Demon Knights and the Illithids might prove to be a challenge, but with proper strategy it shouldn't prove too difficult. and the Beholder lair will be a peace of cake if you pick up either the Shield of Balduran or Cloak of Mirroring.
so, if you're planning on using Imoen after you get her back, I'd suggest get your party to the Spellhold ASAP. otherwise it really doesn't matter when you leave and you might as well enjoy the daylight while you can [img]smile.gif[/img] .