That works for some purposes but it is not truly random so it would be better to use a dice or other more random number if available.
Of course, be realistic with getting random numbers. If the situation calls for a quickly thought decision, that works. If you have dice in your pocket go ahead and pull them out.
You can generate a random number in your head by generating several numbers unreliably and taking the sum mod X.
That works for some purposes but it is not truly random so it would be better to use a dice or other more random number if available. Of course, be realistic with getting random numbers. If the situation calls for a quickly thought decision, that works. If you have dice in your pocket go ahead and pull them out.