Here are my personal guidelines, for your consideration:
1. Some sources' outputs are inherently balanced -- MC phono cartridges, and D to A converters are two fairly universal examples. So if you have balanced inputs on your preamp, you should try to use a phono preamp with balanced ins/outs and if your DAC has balanced outs (most good ones do now), you should use them (rather than the DAC's RCA single-ended outputs.) In fact, I'd go so far as to say that if one is using such sources, it almost behooves you to employ a preamp with balanced inputs, (my opinion of course.) In both cases (but especially with MC cartridges) the balanced out-to-in connection will provide a signal strength usually around twice that of a single-ended connection, resulting in a commensurately lower noise floor (blacker background.) Also, with both of these aforementioned sources, the balanced outputs are the 'natural' outputs, and it is the single ended outputs that require additional circuitry.
2. For the preamp/amp connection, you simply have to decide which tail will wag the dog. If both units have balanced outs/in respectively, then use them. As someone already mentioned, whether or not the actual circuit design of the components themselves is balanced or not is more important for signal processing/amplification but the type of ins/out the manufacturer provides (or not) will determine your choice of interconnect. In other words, if either one (but not both) of your preamp or amp has only single-ended connectors, then you will be limited by that component to single-ended interconnects.
3. One valid (non-neurotic) reason for wanting balanced IC's between preamp and amp is wanting to keep the speaker cables short (especially when using tube amps -- to get the most out of their limited damping capability.) Often, this means long IC's (more than 10 - 12 feet) between the preamp near the listening position, and the amp(s) near the speakers. I've used up to 30 foot cables both BAL and SE between preamp and amp and the only obvious difference was the amp didn't have to 'work as hard' to produce the same SPL when using the BAL. In other words, the sonic character remains the same, but (depending on the amp) the distortion may be reduced with BAL.
4. A word about Cardas (or any other finely stranded) cable over long lengths: the accumulated capacitance (in the case of single-ended IC's) or accumulated inductance (in the case of balanced IC's) could have a profound effect on the frequency balance at the amp end.
5. A word about the minimum one-to-ten impedance ratio convention between preamp and amp: usually it's not a problem with today's equipment, but if you use adapters external to the component in order to use an XLR cable from a SE preamp output to an XLR amp input, it could (as in no. 4) have a profound effect on the frequency balance at the amp end.
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