When you purchase new domains, be sure to use an appraisal system that allows you to view the monthly popular searches for the keywords. I wouldn't recommend reversing the order of the words because the Search Engine Optimizer usually sparse the words out in their sequential order.
For example, a domain registered several years back is now worth thousands. You want to gain access to that name, but all the versions are taken. Such a domain like grandmother.com may be worth a fortune. Take for instance; you may assume the reverse order will give you access to the same domain name.
If you develop MotherGrand.com, you may not have much success, well unless you have a marketable idea. On the opposing side, the appraisal system fails to generate a value on this particular name because there is no such demand for the words in that order.
However, if you are lucky enough to find any variations of grandmother, then you will yourself a nice investment. There are millions of search for grandmother. Sometime the reverse order will work for some variations; that is if the words are popular and universal in order. Otherwise, try to put the work in to find common words.