when is chumra justified? - GR"A

Aside from the question of whether and when appropriate to teach a shitas yachid or chumra, the more fundamental question is whether or when a person should taken upon him/herself such a chumra. Chazal in the Yerushalmi call one who is patur but does a mitzvah anyway “hedyot”, and some chumros are placed off limits as acts of yuhara. So when is chumra acceptable? The Mishna in Brachos (1:3) records a dispute between Shamai and Hillel over whether one should lie down to read shema at night. R’ Tarfon acted in accordance with Shamai and is criticized in the same Mishna, and the gemara debates whether one who acts in accordance with Shamai even gets credit for fulfilling the mitzvah of shema. Why should Chazal express such ire at someone who simply goes the extra mile to fulfill Shamai's view by lying down even though it is not required by Hillel? The GR”A in Shnos Eliyahu rounds up a number of sources and draws a basic distinction between cases where a stricter opinion is rejected as an invalid interpretation and cases where a stricter opinion is a justified interpretation but was never imposed as obligatory. In the former cases there is no reason to adopt the stricter view – it has no theoretical value or justification. In the latter cases, adopting the stricter view is justified, as the validity of the stricter view is acknowledged, it was just never adopted in practice by the masses or imposed as an obligation.