Using the first usable for the router allows you to subdived the IP block later if required without having to change the router IP.
As an example, say you had.
192.168.1.0/27
Network = 192.168.1.0
Broadcast = 192.168.1.31
Usable = 192.168.1.1 to 192.168.1.30 (30 Hosts).
If you make 192.168.1.1 the router and allocate hosts from that IP upwards, you can always decide later to split that IP allocation between two /28s. (assuming you've not gone past 14 hosts)
192.168.1.0/28 & 192.168.1.16/28
If you'd placed the router at 192.168.1.30 and then wanted to split the subnets, you'd have to re-ip the router and all the host config that used it This may not be so much of an issue for a /27 but scale that up to a /24 or /23 and it soon becomes a right royal pain in the ….
It is for this reason that I would always set the router/Firewall/HSRP etc IPs at the start of the subnet block rather than then end.