would visit your plants what ever time you are comftorable doing so there is no specific time or day but you will probably be more likely to be seen during the daytime, seeing how it seems like you are growing in a populated area you might be better off only visiting at night unless you have a good reason to be there to begin with.
outdoors smell is not usually a problem even the most skunky smelling strains are not going to be noticed 100 ft away, someone would have to be very close to them before they would notice the smell.
if its in a low spot you might want to check after a decent rain to make sure its not a spot that floods or has standing water after a rain as plants usually cant survive very long in such conditions.
im sure 5-6 plants would be obvious from a helicopter but trees do help some to block there view, but if they happen to fly right over it they are likely to be spotted.
would start them in the midle of june to the first of july i dont think it would be worth it to start them any later.