It is extremely important to pull the Boot0 pin low during reset for normal operation.
If the pin is left unconnected in a hardware design it will "probably work", but it "might" on occasion detect a "high" on the pin and therefore decide to switch to the internal bootloader.
The button SW2 is of course optional but while the pull-up resistor on the reset line (as indicated) is optional, the pull-down resistor R5 on Boot0 is not optional.
The strength of the pull-down is not very critical. Anything from 10k to 1M will do the job alright.