The Mekor Chayim (by Rav Yaakov Lorberbaum of Lissa, the author of Netivot Ha-mishpat) writes (in siman 445) that one should burn his remaining chameitz on the morning of Erev Pesach immediately after eating whatever chameitz he wishes to eat. Halakha permits eating chameitz on the morning of Erev Pesach until the beginning of the fifth halakhic hour of the day, and requires destroying one’s remaining chameitz by the beginning of the sixth hour. The Mekor Chayim maintained that one should not delay the burning of the chameitz, and should do so immediately after he has eaten all that he has planned to eat, even if there is still time remaining until the point when the chameitz becomes forbidden and must be destroyed. Indeed, some have the practice of ensuring to burn the chameitz early in the morning, in fulfillment of the famous dictum, “zerizim makdimin le-mitzvot” – that one should fulfill mitzvot at the earliest possible time, without any delay.
By contrast, Rav Yosef Karo (author of the Shulchan Arukh), in his Kessef Mishneh commentary to the Rambam’s Mishneh Torah (Hilkhot Chameitz U-matza 1:9), suggests that it might be more proper to specifically wait and burn the chameitz just before the final time by which it must be burned. This way, he explains, one makes it clear that he destroys the chameitz specifically for the purpose of fulfilling the mitzva.
Rav Yitzchak Zilberstein, in his Chashukei Chemed (Pesachim 11b), suggests a possible explanation for the common practice not to delay the burning of the chameitz on Erev Pesach. Perhaps, he writes, since nowadays the burning of chameitz is generally done in large groups, and people bring their siddurim or Haggadot in order to make the formal bittul proclamation (renouncing ownership over all of one’s chameitz), it is abundantly clear that the chameitz is burned for the mitzva. Even if this is done early in the morning on Erev Pesach, it is quite evident that the chameitz is burned for the purpose of fulfilling the mitzva to eliminate one’s chameitz, and so there is no need to delay the burning.
Rav Gavriel Ciner, in Nitei Gavriel – Pesach (vol. 1, p. 218), brings those who suggested that one should delay the burning of chameitz until the beginning of the fifth hour, when the chameitz becomes forbidden for consumption. Namely, burning edible foodstuff before it becomes forbidden for consumption would constitute bal tashchit – a forbidden act of destroying food. As Halakha forbids unnecessarily ruining or destroying edible food, one might argue that chameitz may not be destroyed until the time when it is forbidden to be eaten. And thus, according to this theory, one should not burn the chameitz until the beginning of the fifth hour, the point at which it may no longer be eaten, such that destroying it would not violate the prohibition of bal tashchit.
However, Rav Ciner dismisses this theory, noting that fundamentally, the mitzva of destroying chameitz may be fulfilled already the previous night, and it is only by force of accepted custom that we wait until the morning of Erev Pesach before burning the chameitz. The fact that Halakha fundamentally allows fulfilling this mitzva already at night proves that the prohibition of bal tashchit is not relevant to chameitz on Erev Pesach. Since the Torah specifically commands us to destroy chameitz, doing so does not violate the prohibition of bal tashchit, even if this mitzva is fulfilled before the chameitz becomes forbidden for consumption.