Your designing a webpage? It should be designed for any web browser, not just IE. As not everyone is using IE. You should design it based on the specifications of the language; HTML 4, not based on how one browsers' engine renders it.
Viewable in Any Browser website.
You can test your design as-is with an HTML validator.
http://validator.w3.org/
Also, the same site has vasts amounts of useful information and tools you might want to read and download.
To answer the question, I think any 5.5 would work. the difference between core and sp1 & sp2 would most likely just be bug fixes and not a complete change to the HTML standard/DOCTYPE rendering components