We have experimented quite a bit with having IE9 Compatibility mode turned on in custom SharePoint 2010 Branding.
Out of the Box SP 2010 Masterpages come with the following tag:
<meta http-equiv="X-UA-Compatible" content="IE=8"/>
You may try changing it to IE=9 to get some of the goodies but will quickly notice things blow up and there is simply no choice for the time being except to remain in IE=8 mode.
Recently we tried IE=Edge, which indicates “hey, use the latest IE version”. I guess this could save you some maintenance in the future in some scenarios but really seems like it’s prone to just cause future headaches. If your code outlives IE version iterations you will probably have plenty of scope to evaluate browser compatibilty once in a while.