Depends on your user base and would need to be agreed with your client. For an intranet type application you may be able to avoid it. For public applications - government, banking websites, anything the public may rely on, you need to consider it still.
For more luxury type sites, e.g. 'brochure-ware' for a small business, you may be able to take a call on this, but again depends on your client's requirements - do they want to potentially turn away some business? If you can make it fail gracefully (i.e. still looks ok in IE6, but with less bells and whistles), then you have more chance of selling this.
