Depends on what you want to optimise? If you're referring to gaming, you'll have debates either way, but generally you're not going to get a world of difference between 2600 and 3000 in many gaming applications. If you can get good value 3000Mhz then why not. But personally it depends on your usage. For me I would just go for best value.
If you're editing etc - then that's a different debate. Plus you're also limited by what your motherboard supports.