I usually test Genghis' BG while she is having a bite - not only is it important because of the type of insulin I use for her (i.e. that she has eaten a half hour before I inject), it distracts her nicely and is the only way I can get her reading without stressing her out, which can also spike a glucose reading.
It takes a little time for the food to affect the BG level but I don't think 10 minutes will adversely affect your reading.
The only other thing that will affect your reading is I'm assuming your cat hasn't been "grazing" prior to the test, you mean right when his or her face is in the bowl for the first time in a few hours or more?
Anyone else care to chime in?