As a designer first and a developer second, I would have to say that pixel perfection is a thing of the past. Design is all about compromise and constraints, and one of the best parts of designing for the web is giving control to your user. Your design challenges are no longer about perfect placement, but about creating things that best suit your user's needs. Those needs include catering to your users' browsers, and, most of all, delivering your site to your users in as efficient a manner as possible.
Now, adding a single hack won't slow down your site noticeably, but I'm not sure I like the road this leads down. Percentage rounding is always going to be an issue with browsers until the spec directly specifies how to handle it. This is a huge challenge and one we, as designers, are just going to have to get off our high chairs and deal with.
Your designer should understand this concept and fast. Pixel perfection is not coming back and we should embrace it.