Someone told me to sit for grace after meals (the series of benedictions recited after eating a bread meal). Is that true? I mean, is there such an halachic obligation? or is it, at least, a nice thing to do? And what about other post-food benedictions (not after bread)?
As far as making brachos achronos (after blessings) on food the rundown is as follows:
1) Bensching (Grace After Meals)
With regards to bensching the Shulchan Aruch says expressly in OC 183:9:
צריך לישב בשעה שמברך בין אם היה הולך בביתו כשאכל או עומד או מיסב כשמגיע לברך צריך לישב כדי שיוכל לכוין יותר וגם לא יהא מיסב שהוא דרך גאוה אלא ישב באימה: הגה נ"ל דלאו דוקא המברך אלא ה"ה כל המסובין לא ישבו בקלות ראש אלא באימה מיהו אם לא עשו כן אפי' בירך מהלך בדיעבד יצא [רמב"ם פרק רביעי]:
It is necessary to sit at the time one is Benching, whether he was walking in his house while he ate or standing, or leaning. When he reaches the blessings, he must sit in order that he should have better concentration. Also, one should not be leaning since it shows haughtiness. Rather, when saying the blessings one should sit in awe. Rema: It appears to me that not only the מבורך (should do these things) but the same applies to all the diners; that they should not sit in the way of levity, but with fear. However, if one did not do these things, you are יוצא בדיעבד (by the letter of the law covered) even if you made the blessings while walking.
So it would seem that sitting for bensching is definitely the preferred option.
2) Bircat Me'ein Shalosh / Al Hamichya
In the following se'if, 183:10 the Shulchan Aruch writes:
יש אומרי' שגם ברכ' מעין שלש צריך לאמרה מיושב
There are those who say that also for saying the bracha of 'al hamichya' (said over the five grains) one needs to sit.
The Mishnah Berurah adds by quoting the Biur HaGra that one should specifically sit when saying the bracha achrona.
The Shulchan Aruch HaRav 183:13 confirms the above two points succinctly.
3) Borei Nefashos
Borei Nefashos' lack of mention in the above source is understood to mean that one does not have to sit down for its recitation. However, in keeping with the concept stated above that one's kavanna (intention) is better when sitting there are poskim who say it is better to sit. Refer to the Kaf HaChaim 183:51 and Ben Ish Chai, Chukas 4 who both say expressly one should be cautious to sit with every bracha.