The problem is that this warning tells you something about your data not being what you expected. It could be important or absolutely a non-issue, you did not tell us anything about the functional context.

Ignoring it altogether is not a very good idea IMHO, because the data resulting from the sort will probably still be plagued with these unexpected values.

Filtering out the non-standard values with a grep before the sort is already better, if that fits your goals, i.e. if removing these values does not harm the rest of your process. Another possibility is to decide that these non standard values must always come before (or after) any other values, which means changing your sorting procedure or having a preliminary step before the sort. Another preprocessing strategy might be to replace temporarily the '-' by a very large or very small positive or negative value, or a null value, whatever might set these values asides without having the warnings.

But you alone can decide what to do so long as we don't have any idea of what your are sorting.