Standard format to store text in LSB steganography?
There is no standard. But since the goal of steganography is undetectability, it is not usual to hide data into consecutive pixels. Steganography hiding data into all the pixels is broken.

Some algorithms use a function that detects areas of the image where the insertion produces less distortion along with a PRNG.

See, for example:
https://hal.archives-ouvertes.fr/hal-00541353/document