[GVIrish]

I would first evaluate how much work would be involved with making their design pixel perfect. There are some platforms where making certain things pixel perfect is extremely difficult (Sharepoint customization for example).

I would also give my customer feedback on whether the UI helps them achieve their goals. Sometimes people zero in on a visual design that has poor usability, and all you're doing is taking them further down a bad road.

[andy_adams]

Yes! If you see a problem with the design, share your expertise (politics carefully navigated). This is how you become even more valuable to your client. But if you've settled on a design and you're the implementer, make it look like the specs you got!