Oh gawd NO!!!!
Let the customer give their specifications, don't let them in most of the meetings, or rather don't let them in any of the actual meetings, but have an occasional meeting with them to "touch base".
Set an absolute date for feature lock, PERIOD EXCLAMATION POINT
The more access the customer has, the more they want to fiddle with it, change things, add more features, and completely screw up it's capability to do the job it was intended for in the first place! I've seen this happen...
"I need to talk to the head developer!"
"He needs to make a custom virus definitions so I can get rid of the solitaire app from the network."
--"I'm sorry sir, but this Solitaire is neither a virus nor malware, so we will not assign resources for that. Have you set your administration rules to remove Solitaire?"
"Yes, but my users changed it's name so I can't delete it"
--"You could try office policy to ban the usage of Solitaire"
"Well, then just tell me how to make my own virus definitions."
--"No sir, that would be a huge violation of security precepts, user trust, and industrial secrets."
-- Hearing the customer terminate the call in anger, I check his file and find out he's a troublemaker that often demands insane things. I feel sorry for the employees at his work, but at least they're smarter than he is...