In a dojo it is, or should be, a place of inherent trust in each other otherwise in my opinion it's not a good dojo. Therefore trust should be given as soon as you step on the mat.
There are beginners. There are out of town visitors. There are new students with unknown experience elsewhere. There are other mats, at other dojos.
Yes, certainly I have a great deal of trust in my instructor and the other students at my dojo. Otherwise I wouldn't train there. But blindly extending that level of trust to strangers? No. And even with the dojo, there are degrees of trust: I know them well enough to know the difference between the ultrasensitive yudansha and the junior white belt who doesn't know his own strength.