Having had stuff printed lately, I would just say this:
Get the aspect ratio right for the printing process.
In other words, if you are having cards printed out at 5 x 3 this is an aspect ratio of 1.66
You would be advised to create your art in the same aspect ratio. That way, when it is scanned and sized to the eventual card you don't have stretched characters or shrunken squat characters - they are all kept in proportion
Now, the best thing I can suggest is think about where you will likely get any cards printed.
And know what size you will opt for, so you know what aspect ratio to go with.
They can vary between bridge size (which is ratio of 1.555) poker size (ratio of 1.4) and tarot card size (aspect ratio of 1.727) if you used the websites I used.
Quite a difference between them. So if you created something which has an approx ratio of 1.4 but then decided to with the tarot size, the image would be stretched. And may not as good as you would have hoped.
I would also say you need to know up front if you are going to have borders or not. If you are going borderless, don't have the main image or any important detail going right to the edge of the artwork. When you have it loaded for printing, you are warned that you need to account for a bleed area, so if there is inaccuracy in bringing the blade down in the cutting out part of the process you do not end up with a) something important being lopped of and b) a stray white border appearing on the opposite side.
If you are having borders fine - but again you need to be aware, facturing in a border which would be equal on all 4 sides does change the aspect ratio very slightly. Small border would hardly make a difference. Large border could.
I am not able to create my own deck sadly, but these are things I would sort out before I put pencil to paper. It is easier to know how the end product will look size wise and work to that, rather than spend those precious weeks and months creating a master piece - only to realise you cannot get it to fit the dimensions being asked of you at the printing stage.