Ari mentioned tearing kriyah for an avel, and I remember R'Sobolofsky
dealing with this in his avelus shiurim...
ברכי יוסף says that one is only required to tear a טפח (for a מת) and
tearing any more constitutes בל תשחית. Many, however, disagree with
this because it is hard to imagine that we are supposed to be so
exacting in that one is not יוצא if he or she tears less than a טפח,
and violates an איסור if he or she tears more. Generally, R'Sobolofsky
says, that we assume that the אבל should aim to tear at least a
טפח--and to make sure that he or she does--and if he or she tears a
little more, it's fine. BUT, one should not tear בגדים that he is not
required to tear, or even make multiple tears on the בגד that he is
obligated to tear.
Interestinglty, ש"ך יו"ד תב:ג says that although it is not generally
allowed to tear unnecessarily, one can tear more (and it's allowed)
for אב ואם. This means that even though מעיקר הדין one is not
required to tear a jacket, he or she may do so for אב ואם, even though
for other relatives it would be בל תשחית.
But, R'Sobolofsky did point out that it's not so clear that "extra
tears" are a problem nowadays. He explains that this is because
destroying something that is going to be thrown away anyways is not a
violation of בל תשחית. Since nowadays nobody repairs and keeps the
torn בגדים anyway, making extra tears would seem not to violate בל
תשחית.
Similarly, משנה ברורה (quoted by him, dunno where it is...probably in
או"ח סימן ח) says that if one wants to switch the ציצית on his בגד, he
should unravel them and not cut them off because doing so would be בל
תשחית. BUT, nowadays, R'Sobolofsky says, where one can buy new strings
for $5 and people generally do not intend to reuse the stings, it may
not be בל תשחית to cut them off.
These last two ideas that R'Sobolofsky said might only be under the
assumption that the איסור of בל תשחית is because there is no תועלת in
the destruction, but if the issur is because of a עצם מעשה השחתה, it
might not matter that בזמן הזה it "has no utility." But it's not so
full-proof, because one could argue that although the nature of the
איסור is the עצם השחתה, still, in the avel/tzizis cases, they arent
considered ראוי to be classified as something which is being
"destroyed."