What is the rule for writing number percentages in sentences that are less then 10 percent?

Answer 1

No other standard rule: Experts don't always agree on other rules. Some experts say that any one-word number should be written out. Two-word numbers should be expressed in figures. That is, they say you should write out twelve or twenty. But not 24.

Percentages and recipes. With everyday writing and recipes you can use digits, like "4% of the children" or "Add 2 cups of brown rice." In formal writing, however, you should spell the percentage out like "12 percent of the players" (or "twelve percent of the players," depending on your preference as explained above).

Source: www.dailywritingtips.com

I would say that there is no specific rules to writing number percentages in reference to less than 10 percent that would not apply to all percentages. The percent sign (%) is the only concern with formal writing. The numbers are probably more of a personal preference. I prefer the look of most items spelled out like ten percent rather than 10 percent in general; however, if you need emphasis or need to draw attention to the "10" then I would suggest writing it as a number. Since their formatting is so different from what is around them, numbers pop out more than words would in a sentence.