In terms of production, white type doesn't have to be white in the software. You can use any color and simply designate it to the production house that it is to print white. You can output a solid spot color (any color) or 100% cyan, or 100% yellow, or 100% magenta, or 100% black, then tell the pressman to use white ink for that color. It's the ink in the press that determines the color, not always what you see on screen.
For a 1-color t-shirt, you can design black on a white background... then tell the production company to print white on black. Or, when outputting, output a negative of the artwork. It is rare than a designer actually uses white as a color in such cases. Not simply due to difficulty seeing it, but there are other concerns with white objects in some software upon output (Illustrator tends to set white to overprint making it disappear).
The color separation process in printing makes white not necessary in most instances. It's only on rare occasions that packaging or specific printing methods will need a separated plate for white specifically. However, even in those cases, actual white isn't used. A color plate designated as white is used. For a basic understanding of the printing process and color separation you can see HERE
However, if you really want to use white, you can draw a colored box on a layer below the artwork and set the box to non-printing so it won't print. In Illustrator the non-printing option can be seen in the Attributes Panel with the object selected. Of course, then you are going to print white type on white stock/paper so that won't be seen either.
(A PNG is often unacceptable for print production. And if working in Illustrator a PDF, or even EPS, would be better quality.)