Of course, if it's followed by a valid identifier character, I'll add braces: "${basename}_$num.txt"
I'm pretty inconsistent when globbing: "$HOME"/docs/* or "$HOME/docs/"* are common for me.
I don't use "${HOME}" unless I actually need the braces. Partly it's because the braces are even less necessary in Zsh: $#array[3] works as you'd expect.