Form 12b???

As per Rule26A.

The assessee "MAY" furnish Salaries due or received by him from the other employer to curreent employer.......

Question is,
whether is it necessary to furnish previous employer's salary to current employer for TDS purpose,

if not,

then will it be any non-compliance if employee has submitted last drawn salary slip at the time of his joining & we are not deducting tax on his prevoius salary becoz he has not submitted Form 12B?????