I know that you have awarded an answer, but I just wanted to contribute my "two cents" worth.
In a formal accounting or tax document, the term "tax-exempt" would probably be more appropriate. There is also a subtle difference in meaning. In the case of a donation, for instance, the donation itself would probably not be taxed in any instance. There is not typically a tax placed on the act of donating. So, calling a donation "tax-free" probably does does not convey much information. However, in the tax system of the United States and perhaps other countries, a charitable donation may be subtracted from the taxpayer's income when calculating tax due. This creates a "tax-exempt" portion of income. In this instance, "tax-exempt" is much more meaningful than "tax-free".
I hope I haven't created confusion rather than shed light on a matter which you had already decided.
One more note: whether you use "tax-free" or "tax-exempt", please be sure to include the hyphen. I admit this is a fine point of English orthography but it should probably not be left unsaid.
MBA, experience (on the "giving" end) with the United States taxation system
18 years as professional translator
| Alan Lambson|
Local time: 11:28
Native speaker of: English
PRO pts in pair: 114