I personally find that it is occasionally convenient and natural to start sentences with a mathematical symbol, but I have had coauthors who do not like it, and in this case I typically modify the sentence (into what I sometimes feel is more awkward).
Tara has mentioned a nice trick for amending sentences starting with a symbol and this is typically the approach I have taken. Another approach that I find sometimes works is to consider merging the sentences (perhaps with a little restructuring) so that the symbol follows a comma or a conjunction such as `and'.
Edit: I should point out that I am an engineer, and like most engineers I am good at modifying (some might say butchering) elegant things into `practical' things. This is no less the case with language and might go some way to explaining my relatively relaxed attitude towards starting sentences with a symbol. In my opinion, if it works and is clear, do it. I can envisage situations where starting a sentence with a symbol makes the text confusing to read. Obviously it should be avoided in this case.