Please check https://en.wikipedia.org/wiki/Ethernet_over_twisted_pair
If you have more bandwidth (in MHz), you can transfer more data through a single cable or over a longer distance. That is nicely illustrated on the Wikipedia page:
So, if you are 100% sure that you won't use anything above the Cat5e specifications (and have your cabling certified so it really meets the specifications), like only 1000Mbps, I don't see a need to invest in higher class cable. As with time the Smart Rate equipment costs will come down, if you have invested in Cat6 or better, you can get the higher speed or over longer distances than with Cat5.
In general, the additional investment is much smaller than a wire replacement later on, but its up to you to make up the balance. See it as part of future proofing.