The léi reading shows up almost exclusively in two compounds:
累累 (literary, 'clusters of') and
累赘 (everyday, 'cumbersome / a drag'). Outside these, you will rarely meet léi alone. The four-char idioms
伤痕累累 and
负债累累 keep the lěi tone — only the standalone reduplication
累累 for piled fruit / scars takes léi.