When and why should you write numbers less than 10 in words? My current strategy in my thesis and in papers is to write numbers less than 10 in words. For example, if I need to write the number 6, I write 'six'. Recently though, I've been thinking this strategy adds unnecessary complications. Is there any particular reason why numbers less than 10 should be written in words?
Many style manuals will have specific requirements regarding when to express a number as numerals or in words.

APA Style

The APA version 6 style manual has an entire section starting from 4.31 on how to present numbers in text.

The manual does include a general rule that numbers 10 and above should be expressed as numerals, but there are a wide range of exceptions. 

Should be expressed as numerals


Any number in an abstract 
Any number immediately preceding a unit of measurement  (e.g. 3 kg)
Any number representing a statistical operation (e.g., 4 times as many people)
times, dates, ages, scores, and points, exact quantities of money, numerals as numerals
numbers that indicate a place in a series (e.g., page numbers, Table 1, Table 2, etc.)


There are also further exceptions for when numbers should be expressed in words


Numbers at the start of a sentence or heading
Common fractions
Where the usage is common


Why have the general rule to express numbers below 10 in words?

Compare the following:


I had 2 apples. 
I had two apples.


and 


I had one-hundred and thirty-two choices.
I had 132 choices


A few possible principles:


Numbers less than 10 are easy to read as words; Large numbers are difficult to read as words and easy to read as numerals.
In general, conventions make life easier for writer and reader.
There may be a slight disruption to the look of and reading of prose when interspersing numbers with words.
A few numbers, particularly "1" (which looks a bit like "I") might introduce slight ambiguities.