That depends on a lot of stuff. If you run everything through the pa that does what putting a mic on it is for. I don't think it would make sense both plugging into the pa and putting a mic on the same amp. Generally it's a good idea to go through the pa but if your amp doesn't have an output jack that sounds good then you'll be putting a mic on it or playing straight out.
As always, depends on your equipment and the venue.
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