Critter offers some excellent advice. I wouldn't be too concerned with providing your tax ID and making it readily available. It's not like a social security number and there is nothing I can think of that a criminal could do with it. It's not needed to gain credit or access to a bank account and if they wanted to "steal" an orginization's identity to fool supporters they would pick large, brand name groups like the Red Cross, MDA, American Cancer Society, etc.
The key is not to fool any business..you want to give the correct information.
If your PTO is federally-registered 501(c)(3) non-profit charity, then you should say that in your donation request letter. Wording something like:
"XYZ PTO is 501(c)(3) non-profit organization." You could add more about the extent of deductibility, but there are specific rules that define what a donor can actually deduect as a charitable contribution, so I would just stop there. Let them draw their own conclusion.
Later, as follow up and thank you for the actual donation, your letter should include the item they donated and the PTO's EIN (tax id number). You don't need to specify the value of the item - that's the donor's responsibility.
Remember that donations to your PTO are considered charitable contributions only if your PTO is a 501c3. I've heard of PTO's innocently giving the impression that the donation is tax-deductible, but their group is not 501c3. Also, using the school's tax id number for donors is innappropriate if the PTO is accepting the donation.
I'm pretty much new to this myself, however, I am not sending out my donation request letters with it. I've noticed that many big companies, then have you fill out a form with the tax id request on it. Small local companies, I send it out on a receipt with thank you note. I just don't think that is information who should just give out willy nilly. Especially since mail does tend to get lost and misdelivered. These days, you just never know what some unscruptlous person will do with it.
I am sending out my donation requests without the tax id number listed. Should I be listing this on my letter or should I just indicate that the tax id # will be provided upon request?