I do not think it matters, either writing or speaking. First, the distinction is to an accent, and does not involve entirely different word meanings. If you can manage the use of both accents, it is not a matter of what you can or cannot do so much, (as your question is composed) but whether or not there are persons who would criticize what you do. There is a vast difference between what you can do, and what you are criticized for doing. I know that in the United States, people do not care about "accents". There are people in the USA from all over the world, and people just do not care about what "accent" you have when you speak. I know that as for Accents such as British or Australian, most people in the U.S.A. think them to be very charming. In fact, many Americans like to imitate a British or an Australian accent. Should you choose one? I do not want to tell you what you "should" do. You should do whatever you want to do. If you mix accents it may be "weird" or "funny". But so what? Accents only show up in speech, not written English.

Thank you for all ur comments :)

It will look sloppy and incorrect, as if you can't spell or something. If you use Microsoft Word, you can set the language for your document and you will get different correction alerts based on whether you select English AUS, UK or US. Pick one and do your best to stick with it.

No, don't mix them. Inconsistent spelling/style just looks careless.

if you do that, i think it is too difficult to convey what you want to tell others.