[Help-bash] When are double quotes necessary?

[adrelanos]

Just an example.

temp="/etc/"
file="name"

a)
config_file="$temp/$file"

b)
config_file=""$temp"/"$file""

c)
config_file=$temp/$file

d)
i=0

e)
i="0"

First, a) vs b). I like the look and feel of a) more. Is b) necessary
and/or advised in this case?

Actually, c) should work as well, but it is not advised, because if
temp="/etc/ ", this could be end in a disaster just of because an
extra space.

For trivial cases, such as d) vs e), are quotes still recommended?