I agree with lewm that cartridges are not inherently balanced (only 2 terminals), but a phonostage input can nevertheless create a balanced situation by providing the common mode reference (ground). This can be done using a center-tapped primary SUT with center tap to ground, or by using balanced 24k resistors to ground. Such inputs force the terminals from cartridge into positive and negative polarities with respect to ground. It's a nice way to reduce hum if proper balanced cabling is used.
In most systems, running the two terminals from a cartridge in single-ended mode is reasonable by typing one of them to ground. If you have no hum getting injected, you're doing just fine.
My new ARCHIVER phonostage does this. All of the internal circuitry is balanced, including XLR outputs.