Although some devices allow it and others don't, in general, it is always wise to keep static IP's outside of the DHCP range. On my home network, I start the DHCP range on my router at 50 (xxx.xxx.xxx.50). That gives a lot of room for static IP's, and you are unlikely to need more than 200 DHCP allocations.
Whether or not reserved IP's work reliably will be dependent upon the router - and has been pointed out already, most ISP routers are not very good. I tend to avoid them for that reason. Your ASUS router is an excellent device, and ASUS firmware is pretty good (the Merlin builds are a little better.) You don't have to bother with DD-WRT on that box unless you need a specific feature only available in DD-WRT.