Is it likely that Voldemort or Thicknesse changed Ministry of Magic policy on Obliviating Muggles?
For the time being? I would guess not, based on the following reasoning:
Voldemort isn't very interested in the legal side of matters. He didn't get a Ministry post for himself - he was a very strong wizard, but not a politician. He didn't really need to make the laws, he was strong enough to break them if he wanted - so for convenience, he left lackeys Thicknesse and Umbridge to run the Ministry while he concentrated on crushing his opposition, because direct magical confrontation was his real forte.
The Ministry is a monstrous maze of bureaucracy and apparently one of the biggest employers of the Wizarding world. Aurors, Obliviators, all kinds of troubleshooters, paperworkers, officials... Managing a structure that large is tricky enough - changing existing, working procedures, protocols and processes would've required significant effort even if everyone was too scared to resist - and a change as huge as dismissing the Statute of Secrecy would've possibly been quite destabilizing to the Ministry and the Wizarding world as whole. So I believe it is more likely Voldemort had no plans to immediately retire the Obliviators - they weren't in the way of Voldemort's immediate goals and could be kept out of trouble by keeping them busy with their usual business.
What about in the long term, in the hypothetical scenario that Voldemort would've stayed in power?
In that case I believe Voldemort would've eventually abolished Obliviating muggles, and openly declared his (and the Wizarding world's) existence to the world at large. It matches his character - he loathes muggles and loves to flaunt his extraordinary powers - he would probably feel humiliated hiding from them, and eventually make a forceful public entrance to the muggle scene. After this he would have no longer need for Obliviators as muggle-foolers, and would probably relocate them to other tasks as he saw fit.
EDIT: As TheDarkLord himself pointed out in the comments, the Ministry did do some bureaucratic heavy-lifting by creating a new organ for itself, the Muggle-born registration commission. I believe the reason why such an instrument would have priority over other issues has to do with two reasons: ideological and strategic.
Voldemort's main ideological issue was advancing his racist agenda. Since the issue was very important to him, right there and then, it's likely he urged Thicknesse and his other lackeys to create the commission as fast as possible. Such a move likely had a huge impact on Voldemort's supporters' morale, because outlawing "mudblood" wizards was a precious dream to many of them.
Creating the commission was also very sensible as a strategic tool, because it served as a handy excuse to weed out potential resistance while also instilling a convenient sense of terror over the Wizarding world. It was basically an inquisition that could track anyone to keep them in line and dissuade any internal resistance against Voldemort's plans and the official version of the Ministry's agenda.