Please note that all CSS hacks and JS tweaks targeting IE6-8 were dropped from the codebase as of v0.9.7. So it’s possible that some layout issues might appear in these browsers (particularly in IE6/7). If you need to support these browsers, you may opt for v0.9.6 which was the last one to fully support them. As for rounded corners, border-radius is not supported in IE < 9 so there is no chance for it in older versions.
But, in any case, if you like, please post some kind of demo that I could test since it's impossible to guess what exactly might be causing the issue without being able to test your code.