Hi all -

I found the recommendations from the American Association of Feline Practitioners/Academy of Feline Medicine. Those are the ones I will go with here. FIV and FeLV have different guidelines.

Feline Leukemia
Link: http://www.vin.com/mainpub/feline/aafpfelvdx.htm
Summary:
• Kittens can be tested at any age
• Vaccination with FeLV vaccine does not interfere with the FeLV test

Initially it is given to kittens after 9 weeks of age, and reboostered 2-4 weeks later. If the second vaccine in the 2 vaccine series is given greater than 4 weeks after the first, an additional vaccine needs to be administered 2-3 weeks later. After the initial series, yearly boosters are given. If your cat goes outside frequently, we recommend yearly FeLV testing along with vaccination.
No vaccine can be guaranteed to be 100% effective, so we recommend separating any FeLV positive cat from FeLV negatives cats, even if the negative cats are vaccinated.
*******************
It is vital that a vaccination certificate is not accepted in place of a negative FeLV test.
*******************

This is the part I think that vet may have been referring to:
The blood test itself is quite accurate, but not perfect. Because cats can be transiently infected, it is possible that a cat will initially test positive for FeLV, and then recover and test negative at a later date. This is especially likely to occur in otherwise healthy kittens. In most healthy cat populations FeLV is quite uncommon, and this leads to an increase in the relative number of false positive results. In some studies, over half of FeLV positive results obtained by veterinary hospitals from healthy cats were incorrect.

The blood test may also falsely identify recently infected cats as negative. To be absolutely certain, cats must be tested 1-3 months after their last known exposure. False negatives are more common when samples other than blood (e.g. saliva, tears) are used and when multiple samples are pooled.

Unlike FIV, testing for FeLV may be performed at any age. As mentioned, infection may take up to 1-3 months to develop, so results in young kittens are slightly less reliable. Samples should be tested individually; testing representatives from litters or pooling samples significantly decreases test accuracy.