I would make her to always serve first in matches. A player with her serve should always choose to serve first.
Nice suggestion. And I know what you mean. But I don't think Petra really needs to.
If you look at most of her matches, Petra usually breaks her opponent early, and puts constant pressure on their serve.
Allowing her opponent to serve first, also allows Petra to ease into the match (i.e. get warm), instead of starting off right away, making mistakes off of her serve.
With the exception of the Masha match at the Australian Open, Petra usually starts her matches very well. Win or lose. Usually when Petra loses or has a tough match, it's from the hiccups and siesta's she'll take during/starting from the second set.
Now after saying all that, you are on track discussing Petra's serve.
What I would wish for Petra (expecially at times like this) is, she always be consistent on her serve from the moment the match starts. I know she held serve 77% of the time last year, and to start this year again. But it always seems Petra can do better (many matches she'll have 1-3 silly service games). And she can.
I hope she matures and realizes this year, that her serve should always be a constant, right of the bat. Of course, some of her silly mistakes, and at times over aggressive nature (off the ground), have something to do with that as well. But she needs to be lights out on her serve, from the start, 85% of the time.
And she needs to follow up, many of those good serves, by moving forward/going to the net, to end the points quickly, and prevent herself from making mistakes; especially a time like now, when she's minus match play.