Power Transformer with Three Secondary Windings Question

[apzx]

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[ccaudle]

Would number one work

Yes, that is common.

is there some reason that I may not want to use it?

Depends on what the third winding is used for. On tube circuits sometimes the heater supply is treated differently, or with phantom power supplies you might want to explicitly note that it is connected to the pin 1 connections of the input connectors, not just some random "ground" location that is convenient.
But for solid state circuits the first style you have noted is common when you have for example +/- 15V for analog inputs, then a +5V for logic or ADC chips or front panel microcontroller, etc.

 

[apzx]

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[ccaudle]

This winding will be driving relays.

Then I would not "ground" that winding in the sense of connecting it to the return path for any other circuitry. You can still tie one side to the chassis if you want to have a known voltage level (e.g. if it floats to a high common mode voltage because of capacitive leakage), but relays have big current pulses that you don't want to get mixed into any of your audio return currents.

[edit] Not that I think about it a little, there may be benefits to making sure that the supply for the relay is at a known voltage, but you might want to experiment with the relays to see if there is any feed through from the coil to the signal contacts, and if so whether it changes depending on whether you use +24V/0V, or 0V/-24V for the coil supply. The problem with just showing a "gnd" symbol in the schematic is that it matters a lot where that connection is physically made, so you end up needing to put explicit notes in the schematic about where that connection needs to be made. If you just label it as relay+ and relay- and treat it as an isolated supply you avoid a lot of that headache.

 

[apzx]

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