I use class A IPs with incorrect subnets all the time. Never really had issues. One of the things that the subnet really does is limit the number of IPs available. So....for example. 255.255.255.252 will allow only 2 users on the network.
https://serverfault.com/questions/87...rs-on-a-router
That may explain it a bit better. The MAIN reason I can think of with using the proper class/subnet is to adhere to auditing. If you've got a home or small business then it really doesn't matter.