We will start with a look at what could possibly be wrong with blowing shofar after shul on Rosh Hashana.
It is Rabbinically prohibited to blow shofar on Shabbat,[1] and this prohibition presumably applies to Yom Tov as well. Thus, if not for the mitzva of blowing shofar, we would not be allowed to do so at all on Rosh Hashana. The Rama[2] therefore rules that an adult is not allowed to blow shofar “for no reason” on Rosh Hashana. What does “no reason” include? The Tur[3] cites an opinion that goes as far as to say that one may not blow a shofar only on behalf of a woman on Rosh Hashana, because she is not obligated in the mitzva. (The Tur himself argues with this view because it is still an optional mitzva for her to hear the shofar blasts.) Another discussion[4] raises the possibility that if, out of incorrect stringency, one makes the tokeiah repeat a blast when Halacha does not require it, he violates this Rabbinical prohibition. Thus, it appears that extra blowing “just to be on the safe side” could be a negative thing. (The Taz[5] does say that the prohibition against unnecessary shofar blowing on Rosh Hashana only applies when it falls out on Shabbat, due to the Rabbis’ concern that shofar blowing could lead to carrying outside without an eiruv. However, his opinion is not widely accepted.)
Another issue is the prohibition of bal tosif, i.e., of not adding on to the mitzvot of the Torah.[6] Indeed, Tosafot[7] wonders why the accepted practice of blowing during the amida (or chazarat hashatz[8]) does not violate bal tosif. Tosafot answers that performing a mitzva an additional time beyond what the Torah requires on the day when the mitzva is to be performed is not a violation of bal tosif. We could apply that answer to our case as well. However, we should note that although Tosafot’s rule is quite accepted, the Rashba[9] disagrees and maintains that bal tosif is violated through repetition of a mitzva’s performance unless there is a Rabbinic institution to add on to the mitzva. Therefore, according to the Rashba, in our case there would have to be some type of Rabbinic mandate in order to justify blowing more than necessary.
What indeed is the reason that some people do extra blowing? One reason is that they are concerned (on some level) that the blowing in shul might not have fulfilled the mitzva according to all opinions, arguably leaving an obligation out of doubt (safek). This idea has precedent. The Torah requires only three sets of three blasts, for a total of nine; we arrive at our minimum of thirty blasts because of a safek regarding how to blow the shevarim.[10] While the Tanna’im considered thirty blasts sufficient to be safe from doubt, there are disputes among post-Talmudic authorities on several points that were once clear. Perhaps covering these opinions is a legitimate need that justifies extra blowing. It is possible to vary the blasts during the hundred blown in shul in order to fulfill the major differing opinions,[11] but some shuls do no variations at all, and few shuls do many variations.
Another reason that some wish to blow after shul is that the tokeiah does not always do a good job of blowing accurately, and the rabbi is sometimes lenient about making him repeat blasts. Some people therefore wish to hear extra blasts just in case the ones during davening were not sufficient. Thus, it is difficult to make a generalization as to whether extra blowing is prudent or close to frivolous.
Our recommendations are as follows: If there is a real fear that the tokeiah did a very poor job, or if he does not do even the basic variation of the breaths,[12] there is significant reason to want to hear more blasts in order to cover one’s bases. (Of course, one should avoid insulting the tokeiah or the rabbi.) Otherwise, stringency is problematic, not only for the reasons noted above, but because of yohara (being “holier than thou”) and/or casting aspersions on what others are doing. Therefore, we recommend to the average person to suffice with the hundred done in shul, which almost certainly cover the needs for nine appropriate blasts.
This being said, one should also not cast aspersions on those who believe in being “extra stringent.” Blowing shofar is a particularly beloved mitzva,[13] and it is not unreasonable to want to cover all of one’s bases on the Day of Judgment. There are poskim[14] who support this approach. Regarding yohara, it can make a difference if one acts openly or tries to be discreet about his practice. This is also less of an issue for one who consistently tries to be meticulous in his observance.[15] We certainly do not want to judge sincere people negatively on the Day of Judgment.