In general, the best practice is to not pre-translate a file in advance. It is better to use the default behavior of Memsource Editor, which will always pre-translate the next segment you jump to (after confirming the previous segment).
This ensures that you get the best match available at that moment. If you pre-translate the whole file in Memsource in advance, it is possible that you could lose some matches that are created between the moment of batch pre-translation and the actual moment when the translator arrives at that segment (e.g. translated by someone else or even translated by the same translator earlier on in the file).
Sometimes it is, of course, useful to pre-translate in advance. An example of this would be when you want to work offline, or when you want to pre-translate just 101% matches. These, however, are less common cases.