I always turn to the Australian Government Style Manual for clarification on matters of writing.
In regard to numerals it just says "there can be no absolute rules to cover every possible contingency. In the absence of specific instructions, the decision whether to use figures or words in particular instances must be made by the editor ...". So nothing is "right" or "wrong". It does recommend words for numbers less than 10.
Generally numerals are used in scientific, technical or mathematical work, and words are used in prose, descriptive or narrative work. But this isn't exclusive (see editor's choice above). I agree that numerals should never be used to start a sentence.
Also, on a totally separate issue, the correct written term is per cent, not percent.