Input / output voltage standards for modular analogue synthesizer
For a VCO, 1 volt per octave is common practice but when it comes to power supplies, it's completely down to what the designer feels are most appropriate to do the job. For instance, it makes sense for the VCO to work from a range of supplies maybe as low as +/-9 volts to +/-15 volts (without noticeable loss of performance or error).

Having said that, I'm very aware that to get the best and most accurate 1v/octave performance you'd probably want very stable +/- supplies - the human ear will easily pick-up up tuning problems if the 1 volt/ octave changes slightly and you have to think about this when you have three VCOs working in tandem - you don't want the relative pitch ratios changing as you ramp up the control voltage or it'll sound a complete mess (been there, bought the T shirt etc..)

+/- supplies tend to be used (rather than a single + supply) because that means the 0V midpoint can be used as the reference for analogue signals and the reference point for control voltages such as what is used for the VCO and VCF.

Voltage controlled filters won't suffer the same issue as non-linearity in the 1V/octave that VCOs can suffer from - they are just filters and not the basic sound generator so a bit of non-linearity won't suddenly sound like a mess as it would with tandem/parallel VCOs.