If you use 6V then you don;t need a voltage regulator
If you are using a microcontroller, no, you need a voltage regulator! Without a fixed voltage, the timer will be all whack, and the ADC won't be consistant.
a voltage regulator needs 2 volts higher to work right. SO a 5v regulator needs 7V coming in.
All voltage regulators have a 'dropout voltage', typically 1.5V. An LDO, or low dropout, can be as low as .15V. I use a .3V LDO on one of my microcontrollers.
1.can i use 5 batteries from the one i got=6v to power whole robot or not?
Yes, if you use a LDO regulator.