It's generally advisable to turn it off if you have discontiguous networks, as it would break routing. If you know that you have networks that are on two sides of a router that could be summarized into a larger supernet, then turn it off. Otherwise, if you have a good hierarchical network design, you don't have to worry about it.
You typically WANT summarization to occur, as it reduces your routing table size, making your routers operate more efficiently. If your network is very small, however, you can safely turn off summarization to be safe and it probably won't be a big deal.
