I can't recall a time I've seen the opposite in print (not to say I haven't forgotten), but what I prefer to read (and what I would write myself) would be old on the left, new on the right for the following reason:
The equals sign should be placed above the double barline between the two measures in question. As a result, the value to the left of the equals sign will then appear above the old meter, and the value to the right of the equals sign will be placed above the new meter. This allows one to easily read "okay, the value of the dotted quarter in 6/8 is now going to be equal in time to a quarter note in our new meter of 2/2."
If there are publishers out there doing it the opposite way, the only way I can think of to decipher it would be if the value on the right CLEARLY matched with the meter on the left and vice versa--like if the left meter was 3/16 and the right was 2/2 and the two values being compared were a dotted eighth and a half note, I don't care which side of the equals side they're on--I'm going to be matching the dotted eighth to the 3/16 and the half note to the 2/2.