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I wouldn't classify using HSRP on like devices as a best practice, although it's often practiced.
Likewise using HSRP on different devices, I wouldn't consider a bad practice.
Much depends on what you're trying to accomplish.
For example, given your 4500X and your 3750E, I would consider the 4500X much superior, in many respects to the 3750E. So although perhaps having a pair of 4500Xs would be nice (it would also allow you to use VSS), maybe you don't have budget for a pair. So, perhaps you're using the 3750E in a compact spare tire role and the 3750E is "good enough" to keep you going, if the the 4500X breaks, until you get it fixed.
Or, perhaps, you want different platforms as your FHRP gateways, to avoid possible similar security flaw attacks and/or IOS bugs, on like platforms.
