"Nine out of 10"

Solution 1:

The Oxford Public Affairs Directorate Writing and Style Guide (PDF) says

Do not mix the two styles within a paragraph when they refer to the same category

and gives as acceptable examples

• At least 8 of the 20 students were not concentrating
• Eight of the twenty students were not concentrating
• In general, 8 or 10 students were present at all three lectures

I think this is sensible.

Solution 2:

Yes, according to the Chicago Manual of Style I have read recently, the numbering should most importantly be consistent. Then, I believe that it is generally advised to use numerals only for numbers 13 and greater.

Edit: The Chicago Manual of Style, 15th edition, 9.3:

In nontechnical contexts, the following are spelled out: whole numbers from one through one hundred, round numbers, and any number beginning a sentence. For other numbers, numerals are used.

Thirty-two children from eleven families were packed into three vans.

9.6 An alternative rule:

Many publications, including those in scientific and financial contexts, follow the simple rule of spelling out only single-digit numbers and using numerals for all others. This system should be used with flexibility so as to avoid such awkward locutions as “12 eggs, of which nine were laid yesterday.”

Solution 3:

Style guides often suggest using words for numbers below ten, digits for higher values. As @Henry points out, mixing style is not endorsed, but increasingly people are forgetting (or never knew) that caveat...