As for me...
I've started the module as a player three times. The four of us were experienced players, and used good tactics. We built powerful 10th level characters that meshed into a single focus. We decided to have fun with Savage Species. We got wasted.
Built a new party with humanoids focused on survivability and high level buff spells. We got wasted the second time. The third time we added a 5th player, and went in at 12th level. We were a pretty nasty group. Multiple Evil Clerics, a Troll, an awesome Psion, and a great Rogue. We got to about the same point and found ourselves in a tactical stalemate with no way to proceed without unprofitable casualties (We played an Evil party the third way through so as soon as profit margined dissappeared, we let someone else try to save the world).
Having never got through the first quarter of the module as a player, I bought the module, and am now having my players go through it. They are entering at a significantly higher level than the module expects. As a DM, I am not increasing the HD or number of creatures. I expect the CR system to even itself out pretty quickly.
I would reccommend that you enter with 6 or 7 10th level characters. Entering at level 8 or 9, will cause a few problems.
First, as you can no doubt tell, an adventure called "City of the Spider Queen" involves a fair amount of time in the UnderDark.
Long term expeditions down there (And as 8th level characters, you'd probably know this) require the ability to see in the dark, be self-supportive for long periods of time, (food, water, and sometimes air), as well as some need for flight, or enhanced climbing.
A Party of 7 Level 8 characters is going to be spending most of their initial gold on ways to simply look around, and move freely. Just goggles of night and slippers of spider climbing is a substantial investment for an underground jaunt.
Whether or not the campaign is in the Forgotten Realms, the default CR for Random encounters in the UnderDark is decidedly more deadly than any other environment. In Encounters in Faerun, no other environment on the planet has a CR ceiling over 13. The Underdark's ceiling is 20 for Random Encounters.
I'm not advocating Pit Fiends as Random Encounters, but a party of 7 Level 8 characters has enough experience that the Underdark is dangerous. Really dangerous. There's all sorts of aberrations and humanoids down there, most magically infused, and resistant to most magic, and are probably beyond their abilities to take on for extended periods of time.
At tenth level you've probably just got enough power to _think_ you can take it.
Your DM probably doesn't have to add creatures to beef up encounters or start you at a lower level of HD to challenge you. All he needs to do is play the bad guys as extremely cunning and tactical, and he can kill you over and over and over.
My DM did that to us over and over again. I'm going to do it to my players. And your DM will do it to you.
Having a party of 6 or 7 gives you some flexibility, but it also does handicap you in certain ways.
The adventure is listed as progressing characters from 10 to 18. With 7 characters, you possibly get to 17. I say 17, because eventually as you proceed, you'll fall behind XP bit by bit, and eventually there will be more and more levels lost due to character death.
The end result is that at the end of the module, your party might not be able to cast a ninth level spell, whereas the module was built with the expectation that someone might be able to and thus planned accordingly. That's not to say that you NEED to cast ninth level spells, but the the BBEGs were built to withstand ninth level spells, so why do you think your puny 8th level spells will do the trick?
Another way you'll be handicapped is in the expectation of a acceptable casualties. A DM with a larger party (And multiple Clerics or Druids) isn't as afraid to not pull punches when it comes to character death. So you might have a more grittier experience than with a 4 person party, where a character death is an unacceptable loss of resources; whereas in a 6 person party, the party may "press the advantage" if only one person dies, in the right situation.
Finally, remember that you'll be splitting treasure 7 ways instead of 4 ways. This means you'll have less resources for each character to survive. In higher level campaigns, this is a big deal. This would only be exacerbated by starting with level 8 characters, making your party even more undertreasured as you went along.
I say stick with level 10 characters.
As a final question, will you be running 3.5 or 3.0? I say this as I have a 3.5 campaign running and must note that the listed tactics for the villains will be severly altered by changing to 3.5. Your DM may want to just say that whatever planet wide cataclysm that changed the way magic affects denizens of the planet, didn't reach all creatures native to the underdark.
Trust me, much less headache for your DM to convert. Doesn't have to be universal to all the 3.5 changes, just certain spells. I'm sure you can guess which ones.
