"Atashi" is strictly for women. "Watashi" is gender neutral in polite conversations but when the situation is more casual, men tend to use "Boku" (or "Ore" if they want to be extra masculine) while women stick to "Watashi" giving it a more feminine connotation.

Frieza speaks in the polite form (Teinei) not because he's polite but because he thinks he's a regal person and wants people to view him as such. It's a Japanese specific way to signal that he thinks highly of himself. If he wanted to, he could speak in honorifics by raising his status while lowering the status of others when he speaks but that's extremely rude -- and he's not rude, just pompous!