When I was in college, I was told that I had to stick with only either present tense or past tense throughout the essay.
Is it still true for AWA?
Thanks in advance.

You should generally stay in the same tense unless you have a reason to change. You will likely need to use both tenses, though, for the Issue essay (at least). When you are stating some maxim you believe to be "always true" then it should be in present tense. When you are giving a real-life example to illustrate that maxim, it will have to be in past tense (because whatever the example is, it is presumably not actually happening right as you take the test). :)