Hrih is the Seed syllable of Amitabah of the Lotus family, and consequently also of Chenrizig. Hrih is very distinctly in the middle as all the other syllables revolve counterclockwise and around it.
This being said...Nyingmapas generally DO NOT, in the majority of cases, say the six syllable mantra with the seed syllable HRIH at the end when reciting the 6 syllables...otherwise it wouldn't be the 6-syllable mantra.
What would be more common and most advised and practiced would be to say HRIH as the end of each mala round thereby reserve saying HRIH for the Guru bead, which is what my Sangyumla told me when I was starting out.
((by the way, Hrih is also reserved in the Amitabah sadhana and said Exclusively on its own for at least a mala round or more in the dissolving/completion stage of the practice ....for those of you wondering and desiring to maximize your Hrihs))
With regards to OM MANI PEME HUNG...It would probably be more beneficial to say the HRIH evey time but honestly, to me, it just sounds and resonates funny when staying the basic mantra with some good speed!
By all accounts always say HRIH on the guru bead when saying the mantra