Most trailer wheels have zero (or very near zero) offset and so that's what most trailer axles have been designed to go with.
If you buy heavily-inset wheels (where the wheel/tyre centreline is set in towards the vehicle's centreline from the wheel mounting surface), you need to check that the inside of the rim will clear the trailing arm on the axle.
Here's an example I did for 'feralxjsc'/Robert showing a 2" inset Jeep 16x7 wheel on a Dexter axle:
Maybe Robert will read this and tell us whether he ever got confirmation from Dexter that this would or wouldn't be a problem.
If you go for a outset wheel, you won't have these difficulties, but you will run the risk of making it impossible to get the wheel/tyre near to the trailer sidewall. This diagram from another thread shows a zero-offset wheel on a minimum-overhang Flexiride axle - if you added 2" of outset to the wheel, you would have a minimum of 4.1" between the frame and the tyre.
I'm sorry if this lot just confuses you, but these are problems you can come up against, particularly if you go choosing wheels on appearance rather than dimensions! If you limit your choice to wheels between 1" outset and 1" inset, you should be OK.
I've said it before, but to help understand all the dimensions on an axle, this pdf from Dexter is really excellent and should be saved on everyone's hard drive:
Axle Measurement
Andrew
