There is one question. I've made board based on "arduino to breadboard". And my project must do some comm over Serial, but not all the time board is coonected to PC. Do I need to close port when i disconnnect from PC and open it in case of connection?? By the way when disconnected it is powered from li-ion battery with step-up converter from pololu (U1V10F5).
Arduino's job is to send data to the serial port. It doesn't care if anything is connected thereto to receive the data - or not. I don't think the power source is relevant, it might not even be necessary.
I have a breadboard Atmega 328 running 24/7 and sending data to nowhere. Occasionally I plug an FTDI cable onto the board to read the data - occasionally may mean once in 12 months.
Atmega 328s on a breadboard will work fine with the internal 8MHz oscillator when powered directly from a couple of AA cells or a single LiPo cell.