When I'm sick my levels will be slightly higher, so I'd probably not do the test if I had/were getting over an illness. But I have to point out that for me, the numbers might vary 10 pts or so. I"m sure someone will jump on and vehemently disagree w/ me here, but while a 10-20 pt margin may put you over the edge into diagnosis, the GTT numbers are really high. A person w/ normal glucose control just won't get that high.
If I had risk factors, I wouldn't wait a month to test, but of course that's just me, and based on my experience--no risk factors except age and a previous GD pregnancy, healthy weight and lifestyle, and diagnosed in the first trimester. I'm very glad I haven't been running high for the past 24 weeks!