Thank you for your question. PURE alcohol itself is nonconductive, however, that is impossible to find and use in electronics.
When you mention 91% alcohol, the balance of the solution is water, which can be conductive, depending on where you get the alcohol. If there are salts or other minerals in the water, they may cause the solution to become conductive.
Alcohol also will attract moisture from the atmosphere and their again, depending on the environment (pollution), will depend on the salts/minerals getting absorbed into the alcohol.
Let me clarify one thing, in many cases alcohol would be fine, as long as the alcohol can COMPLETELY evaporate. Also, since water, salts, minerals maybe left behind, we always (on electronic/electrical connections) recommend using our DeoxIT® to remove the contaminants and then protect the metal surfaces. If you apply alcohol on a surface that is porous, phenolic-based switches for example, the alcohol/water mixture may get absorb into the phenolic and then the switch may short.
As a review; when using alcohol on electronics if you need to, we always recommend 99.9% electronic grade alcohol (less water and contaminants). If the surface can absorb the alcohol and have the possibility of shorting, we DO NOT recommend alcohol. If the device is non-critical and surfaces have non-absorbing materials, sure, it should be fine. Just make sure you let the alcohol completely evaporate and then treat any electrical connections for protection (to prevent oxidation or rusting). However, if you are using it on a critical and expensive device, do you want to take the chance?