It think your substrate is really a personal choice. Of course if you have a show tank you will want some type of sand or gravel for that finished off look. But the fish don't care, the majority will do fine without it, it is just if you get into working with some of the feather fins, or shell dwellers you may need it to get them to spawn.
Everyones water is different, if your water if fine, than its not an issue. But if you do need to boost your pH a substrate is an excellent way to do it, that way you don't have to add buffer/ph booster, etc. each time you change your water, because your substrate will take care of it for you.
The best place to buy your substrate is it the gravel store or landscape store. Most have jars with samples and you can pick the one you like best!
Cichlid Power!
Pam