If you’re theme is working fine in IE 6, 7, 8 & 9 with compatibility mode disabled then you don’t really need to have it enabled.
If you (try to) respect web standard, you can simply always use http-equiv=”X-UA-Compatible” content=”IE=edge”. To keep compatibility with old browsers, just avoid using latest web features: use the subset supported by the oldest browser you want to support. Do not care about the best IE version rendering.
The positives sides of not having compatibility mode enabled has been discussed here.