The Parashiot of Behukotai and Ki Tavo both contain sections that deal with Hashem rebuking the Jewish people, known as the Tochahot. Due to the harsh nature of these rebukes, Rabbi Yehuda HaHasid (Sefer Hasidim, 766) discussed the reluctance people would have to be called up to the Torah for these particular Aliyot. The solution was to call up unlearned people who did not understand the nature of the rebukes, and not a Torah scholar or the like, lest the curses outlined in the Parasha befall him. Having said that, he writes that one should not refuse to go up if called up for the Aliya of the rebukes because one should always be open to receiving rebuke, as it is said (Mishle 3:11), “and do not hate His [ie Hashem’s] rebuke”.
Rabbi Ya’akov Antebi (Knesset Hagedola, 426) writes that one only needs to be concerned if one was cursed by someone who had that intention. When going up for the Aliyot of the rebukes, however, it is certainly not the intention of the Torah to curse the Oleh, and therefore one need not fear. Rabbi Yoshiyahu Pinto of Damascus (Nivhar MiKesef, Orah Haim, 4) writes that the custom in both Damascus and Baghdad was that the rabbi of the community would specifically be called up for the Aliya of the rebukes.
In Morocco, many had an erroneous practice to seek out an ignoramus and pay him money to be called up for these Aliyot. In fact, in the city of Marrakesh, there was a tradition to call up only a certain family for these Aliyot and they were referred to as the Ben Tavo (sons of [Parashat Ki] Tavo) and all the local synagogues would wait for them to come to receive the Aliya. Baba Sale recounted (Yisrael Saba Kadisha pg 275) that he was once in a small village next to the city of Azrou during the Shabbat of the Parasha of the Tochahot. In their unfounded fear the members of the community refused to take out the Sefer Torah and instead read the Parasha from the Humash. Baba Sale rebuked them and went up to the Torah himself to show them not to be concerned.
Indeed, Rabbi Yosef Benaim (Noheg Behochma, pg. 40-41) writes that although the custom is to seek out an unlearned person, whoever does end up getting called up is praiseworthy and will be blessed by Hashem. The Torah, he continues, is the elixir of life and that nothing bad can come from the Torah.
Furthermore, it is written of the Arizal (Sha’ar Hakavanot, pg. 73c) that he would read the passages of the rebukes in a loud voice as he was not concerned about any bad befalling himself or the congregation. The HIDA (LeDavid Emet, ch. 10), on the other hand, discusses the custom of the Ba’al Kore reading the Aliyot of the rebukes as well as that of the Het HaEgel. Rabbi Shem Tov Gaguine (Keter Shem Tov) explains that the Arizal’s spiritual level was such that he did not need to be concerned about the rebukes and could read the loudly, whereas for the average person there is some fear and thus it is read low voice and this appears to be the custom of many to this day.
Summary: Although some try to avoid the Aliyot of the Tochahot, one need not be concerned if one is called up. The passages of the rebukes are traditionally read in a lower volume than the remainder of the Parasha.