I suppose it really depends on how you're doing it.
You could always get a RecordEnumerator with an appropriate RecordFilter and RecordComparator to look through the RMS to find the one you're looking for. Or, if it won't be too ungainly, you can do what I did with one app, and keep a lookup table of the record IDs in your application (I did this due to the possibility of frequent updates of information, to spare myself having to write searches as I was doing it quick-and-dirty).
But no, you're correct that you can't assume that it's consecutive.