If we are talking about interactive use, you could always use an alias.

But if we are talking about scripts, you should at least use "fgrep -- [a] < file". And if you’re adding an option anyway, you might as well use "grep -F -- [a] < file". Personally, I prefer using options, specifically long options, in scripts; meaning "grep --fixed-strings --regexp=[a] < file".

If you don’t do this, the script will fail spectacularly the day when the string happens to start with a hyphen (-).