According to the Chicago Manual of Style (subscription-based, sorry), you need to use the comma before too "only when you want to emphasize an abrupt change of thought."
Here's an article that gives that quote and has other illuminating things to say on the subject: http://www.dailywritingtips.com/comma-before-too/
Edited to add example from the second link:
He didn’t know at first what hit him, but then, too, he hadn’t ever walked in a field strewn with garden rakes.
In most other cases, commas with this
short adverb are unnecessary.
The question that you brought up wouldn't address your question because "if" is not one of the coordinating conjunctions, but it is a subordinating conjunction. The words that introduce each are:
Coordinating Conjunctions: and, but, or, yet, for, nor, so (a helpful acronym is FANBOYS)
Subordinating Conjunctions: after, although, as, as if, as long as, as though, because, before,
even if,
even though,
if,
if only,
in order that,
now that,
once,
rather than,
since,
so that,
than,
that,
though,
till,
unless,
until,
when,
whenever,
where,
whereas,
wherever,
while
The definition of a subordinating conjunction is:
Subordinating Conjunction (sometimes called a dependent word or subordinator) comes at the beginning of a Subordinate (or Dependent) Clause and establishes the relationship between the dependent clause and the rest of the sentence. It also turns the clause into something that depends on the rest of the sentence for its meaning.
He took to the stage as though he had been preparing for this moment all his life.
Because he loved acting, he refused to give up his dream of being in the movies.
Unless we act now, all is lost.
This article from Purdue on coordinating and subordinating conjunctions explains the usage of the subordinating conjunction as follows:
Notice that when the subordinate clause comes at the beginning, it’s necessary to insert a comma.
From this, the correct punctuation of "You can call me if you need me" is:
You can call me if you need me.
If you were to move the subordinating conjunction to the beginning, however, you would need the comma as follows:
If you need me, you can call me.
Best Answer
The comma is there to add emphasis. It also indicates where a pause might occur in speech, but the pause is as a result of the comma, not the other way around. So, one would pause there for the emphasis that the comma indicates. Despite what some of us may have been taught at school, punctuation should not be used to indicate pauses per se (except in representing how something was actually said, eg, in creative writing or transcription).