I am wondering when it is appropriate to act upon “chumrot” [stringencies] and when it is not. Is it ever forbidden to be stringent even though being lenient would compromise one’s standard of observance?
Following “chumrot” (stringencies) and being “machmir” (being stringent) is a very delicate matter.
For one, a person may not make up his own chumrot except as realistic, personal safeguards against transgressing. But as far as proactive practice or initiating restrictions, one may not add to the mitzvot of the Torah. Therefore, only well-established, long-standing chumrot which have a basis either in halacha or in the valid tradition of a community or family may be adopted.
Interestingly, sometimes halachically-based chumrot are not only allowed to compromise observance, but they actually suspend it. Some examples of this would include chumrot instituted by the Talmudic Sages which preclude observance of otherwise obligatory mitzvot.
For instance, in a case where one is in doubt as to whether he pronounced a blessing, the Sages required stringency regarding making the blessing now and they forbade him from doing so. This is because in the case that he had in fact already made the blessing, doing so now would be akin to reciting G‑d’s name in vain. Rather, in such a case, because of this rabbincally-sanctioned chumra, one must proceed without reciting the blessing, resulting in the negation of a mitzva on account of a chumra.
Another such example is when Rosh Hashana occurs on Shabbat. Even though the Torah prescribes blowing the shofar on Shabbat, the Sages were stringent and prohibited doing so because of a concern that people would transgress a Torah prohibition by carrying the shofar in the public domain. Thus the chumra suspends the Torah-mandated observance.
It is important to note that while in these cases the Sages exercised their Torah-given mandate to safeguard the Torah from possible transgression, they only did so regarding passively refraining from fulfilling a mitzva. But they did not institute chumrot that would involve proactively transgressing a Torah prohibition.
That being said, chumrot which are not required by halacha, even if they are based on halacha, and even if they are ordinarily upheld by an individual, family or community, may not suspend the performance of a mitzva, or be used as a justification to transgress.
An example of the former might involve the chumra of eating only handmade matzot for Pesach and not to eat any machine-matzot. Since nowadays this is only a chumra which is not required by halacha, if one had access only to machine matza for Pesach, such a chumra must not take precedence over the requirement to eat matza. Rather in such a case, one must compromise his chumra, not the halacha. Thus, he must be lenient in his stringency and stringent in his observance.
Regarding the latter, an example might involve being machmir to rely only on top-of-the-line (mehadrin) kosher supervision, even though a standard supervision is available which is undoubtedly kosher. If a person found himself as a guest in another’s home where his well-meaning hosts have prepared food with kosher but not mehadrin supervision, and they would be hurt, offended or embarrassed if he refused to eat what they prepared, it would be forbidden to uphold his chumra and thereby transgress mistreating others. Rather he must certainly compromise his standard of observance and uphold the dignity and happiness of his hosts by partaking of and enjoying what they prepared for him!