How does the position of hydroxyl group in a nucleotide monomer affect the dehydration synthesis of nucleotides?
The phosphate ion is of vital importance in biological systems. Its molecular geometry is tetrahedral. Thus, based on the $\mathrm{pH}$ of the system, hydroxy groups are inter-changeable with $\ce{P=O}$ form and hence all three remaining $\ce{O}$s are equivalent (see Waylander's comment). Therefore no impact on the dehydration.
However, the stereochemistry of the hydroxy group on the 3'-carbon on the deoxyribose nucleus of the nucleotide has an impact on the enzymatic dehydration process. For instance, if you change the original $(S)$-orientation of 3'-hydroxy group to $(R)$-orientation, DNA-synthase would not pick that nucleotide to synthesize intended DNA sequence.