For best-running 00 models:
1. There is not a standard back-to-back dimension. For best results different wheels should be set according to the flange thickness. If you are not sure of which wheels you are dealing with, measure the width of them.
2. RTR wheels will be around 2.8mm wide. Assume the flanges are 0.8mm thick and set the back-to-back to 14.4mm.
3. Romford/Markits wheels will be 2.5mm wide. Assume the flanges are 0.7mm thick and set the back-to-back to 14.5mm.
4. Kit wheels such as Alan Gibson / EMGS will be around 2.3mm wide. Assume the flanges are 0.6mm thick and set the back-to-back to 14.6mm.
5. For 00-SF track, make sure no wheels have back-to-back less than 14.3mm.
6. For DOGA-Intermediate track, make sure no wheels have back-to-back less than 14.2mm.
7. For 00-BF and Peco track, make sure no wheels have back-to-back less than 14.1mm.
8. If you want to use the same back-to-back for all wheels, make it 14.4mm. Kit wheels won't run so smoothly, but they will run.
9. Except for DOGA-Fine track, where wheels should be set in the range 14.7mm to 14.8mm back-to-back. Refer to the DOGA web site for more information.
Adding some detail to that:
RTR models such as Hornby and Bachmann normally have wheels 2.8mm wide conforming to the NMRA RP25/110 profile with flanges up to 0.8mm thick. They should be supplied set to 14.4mm back-to-back, which is also the DOGA-Intermediate setting -- see: DOGA wheel standards
14.4mm back-to-back gauges are available from DOGA shop for DOGA-Intermediate.
Sadly the manufacturers don't always have the best quality control. Most modern RTR models will be found to run fine straight from the box. But there is always the possibility that you will get a rogue wheel set which needs adjusting as above (or ideally returned to the supplier for replacement).
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Locomotive wheels from Markits/Romford are 2.5mm wide with flanges 0.7mm thick. These are the traditional BRMSB "scale 00" wheels which should be set to 14.5mm back-to-back. Using the Markits/Romford squared axles you don't have much choice over this.
14.5mm back-to-back gauges are available from DCC Concepts and Markits
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Rolling stock kits such as Parkside Dundas are usually supplied with Alan Gibson or similar wheels which are either to the EMGS profile or the very similar NMRA RP25/88 profile. These wheels are 2.3mm wide with flanges 0.6mm thick. For 00-SF, these wheels should be 14.6mm maximum, 14.3mm minimum, back-to-back. Ultrascale wheels are similar.
These wheels may be supplied set to 14.7mm or 14.8mm back-to-back which is intended for the DOGA-Fine standard and is too wide for good results on 00-SF, DOGA-Intermediate, 00-BF or Peco. They should be adjusted as above.
Some non-UK "code 88" wheels have flanges thicker than 0.6mm, and should therefore be treated as RTR wheels, as above.
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If you are not sure of which wheels you are dealing with, measure the width of them:
Wheels 2.8mm wide -- back-to-back 14.4mm MAX.
Wheels 2.5mm wide -- back-to-back 14.5mm MAX.
Wheels 2.3mm wide -- back-to-back 14.6mm MAX.