When doing rounding, please take note of the following:
A = price without GST
B = GST
C = price with GST
Formula
A + B = C
Method 1:
B = C - round ( A * 100 / 106)
A = C - B
Method 2:
A = C + round ( B * 6/106)
B = C - A
Method 1 will give you more accurate figure. Method 2 will give you bigger rounding error. I know the difference is subtle, but rounding a number gives you higher precision when the number of significant figures are bigger. (Maths and physics in O level / SPM).
For more background reading: