Even though I am no Microsoft fanatic, I will quote from their Manual of Style for Technical Publications again (which is our Bible at work...it even overrules the dictionaries):
"Numerals vs. words:
Use numerals for 10 and above. Spell out zero through nine if the number does not precede a unit of measure or is not used as input. For round numbers of 1 million or more, use a numeral plus the word, even if the prefix number is less than ten. Use numerals for all measurements, even if the number is under 10. This is true whether the measurement is spelled out, abbreviated or replaced by a symbol. Measurements include distance, temperature, volume, size, weight, points, picas and so on, but generally not days, weeks, or other units of time."
So far Microsoft. This continues for two pages, I just quoted the part that seems relevant to ordinary text as well. Oh, one more item might be of interest here: "Maintain consistency among categories of information, that is, if one number in a category requires a numeral, use numerals for all numbers in that category. When two numbers that refer to different categories appear together, spell out one of them. Examples: One booklet has 16 pages, one has 7 pages, and the third has only 5 pages. There are ten 12-page booklets."
Hope this helps! Regarding the heading: According to these rules, your 12 dead people must be a numeral, but in newspapers that usually depends on the layout, if the article is across one column the heading would be different than across two columns.