My doctor told me that, for consistency, she wants me to get my blood tests done at the end of a patch, just before I change it. I normally change patches Friday mornings and Monday evenings. So, I go either Friday morning before breakfast (especially if she wants to check sugars or lipids) and change the patch when I get home, or shortly before the lab closes on Mondays.
Before I started doing that, I had some pretty high E levels that caused her to reduce my dosage. We realized on the next prescription cycle that the reduction was not good for me, and that's when she realized that it was the timing that was throwing off my levels. Now, I test fairly consistently.