I had this question yesterday, when I was using my navigation system to navigate somewhere completely new in my car. I, by accident, took a wrong turn that resulted in multiple miles of detour. When I realized my mistake, I verbally spoke an explicit word out loud. I was completely alone in the car at the time.
I immediately realized my second mistake in speaking as such and I verbally corrected myself to a non-explicit word.
The Noble Eightfold Path puts emphasis on right speech, and not saying anything to harm others and self. I was not meaning to harm anyone in my outburst, but to express the anguish of a delayed trip.
Buddhism writings (specifically the Dhammapada) also put emphasis on sage qualities, more specifically in the quote "a sage tames himself". Certainly what I did in that situation was not a kusula action, and thus can help breed other akusula actions.
My question is: Is it bad practice to display this kind of poor self-control when in solitude?