Let's Control It

Looking at the ESP8266 datasheet from Espressiff it tells how to connect GPIO-0, GPIO-2 and GPIO-15. From the table you can see that GPIO-0 level should be High at boot, for normal boot.

Hardwiring GPIO-0 to VCC won't work. This should be done with a series resistor with a value about 10k. If hardwired to GND, ESP is in FLASH mode, if hardwired to VCC, ESP is in some other not described state. If connected to a pull up resistor, ESP is in normal boot state.

Can anyone explain why hardwiring to VCC is not working? According to the datasheet this is not correct.

Hardwiring GPIO-0 to Vcc will destroy your ESP when you (accidentally) make GPIO-0 low, which is easy to do with a command like http://<ESP IP address>/control?cmd=GPIO,0,0
So ALWAYS use pullup/pulldown resistors of about 10k when you want to force a GPIO in a certain state.

To answer the question: I believe you can directly connect GPIO0 to GND to get in FlashMode and connect GPIO0 directly to VCC for normal mode.
To my knowledge there is no difference in the bootup state with or without a pullup resistor.