Basically, you should always put dummy devices on the side, few of them, when matching is a concern. Then, aspect ratio shouldn't really matter much. However, every technology has a minimum W that's allowed. Then for a given L, this will define the lowest aspect ratio. I don't have the impression that people really prefer to have square aspect ratios. I think you should have large enough W that you can fit at least couple of contacts. As for the max aspect ratio, increasing W too much will have higher gate resistance, if you care about it.