If your ISP gives you enough public IP addresses for all of your devices, you don't really need to use NAT, and in fact avoiding NAT will probably give you less hassles with using various Internet protocols.
NAT provides a little bit of "poor man's firewall" protection to your home network, by blocking computers outside your home network from initiating connection to computers inside your home network, and by keeping multicasts from leaking out to the ISP (and possibly to your neighbor subscribers). But if you've got some other firewall solution, you don't need NAT for that.
If your cable modem box is already doing NAT, then you definitely don't want your Wi-Fi router to also do NAT, unless you really know what you're doing. And if you really knew what you were doing, you probably wouldn't have needed to ask this question. :)