pronunciation of 着 in 怎么着
Dictionaries generally only show the 'standard' heteronymic readings of a Han character, rather than including all its contextual variations. This is the unwritten standard on Wiktionary as well, on which I am an admin. 

As you have precisely discerned, both pronunciations are acceptable in this particular phrase; however, the 'standard' reading of zhe0 would be used in a dictionary for this particular phrase instead as it applies to a wider range of contexts, whereas zhao1 is restricted to this particular context (怎么着).

See the Wiktionary entry for more information: 怎么着