a or あ, since my fingers are on home-row, and if I hit something like f when it’s asking for the Japanese version, I have to add another input to make it something like ふ. Hitting A will be one input, whether it be in Japanese or English.
Though, with kanji or jukugo, I look at the radicals used, and make a best guess on a common reading for certain compounds. ^^
This is made much easier with this script to help highlight the patterns: