I would also ask similar questions — see if you can spot the trap —
when to tenuki ? when to invade ? when to reduce ?
when to cut ? when to jump ? when to extend (nobi) ?
when to connect ? when to push ? when to hane ? when to connect solidly ?
when to fix with a tiger's mouth ? when to not fix at all ?!
when to attach (contact enemy stones) ? when not to attach ? ...
( the list is almost endless. )
This is the trap: I was looking for some general, nebulous
one-size-fit all answer. Because that would be so convenient !
Learn it once, apply forever, in many ( or ALL ) situations.
There is no such thing in Go.
A generic one-size-fit-all reply: when it's good for you.
Another version: it depends. ( Depends on what, you may ask. Reply: Exactly. )
Other common variations: when the trade is good for you;
when you gain more territory than your opponent;
when you gain more power than your opponent;
when good aji is more important (than other factors);
when it's better for yose (later);
when you cannot lose the cutting stones;
when you should just give up the useless stones;
when eyespace is more important (than other factors);
when no ko threats is more important (than other factors);
... the list goes on endlessly, again.
Adults ask questions like these. Adults love general patterns, general guidelines. Proverbs.
Kids don't (usually) ask these questions.
A teacher would show a double approach to a child; the child accepts it.
The next time, the teacher shows another double approach in another situation; the child accepts it.
Yet another time, the teacher shows when a double approach is bad in another situation; the child accepts it.
Over time, over
tens and hundreds and thousands of different situations, the child gains
experience.
This is how a child learns to walk. The child does not ask, "When do I put my left foot forward?"
"When do I put my right foot forward?" "When do I skip (instead of a regular step)?"
"When do I stop (all together)?!" "When do I step sideways? "
"When do I step backwards?" The child walks, and falls -- rinse and repeat.
This is a perennial discussion -- many many threads, thousands of words (and hours) have been spent
on this and similar topics here on this forum, on KGS, etc.
Some people would raise the point that learning to walk or to talk is fundamentally different than learning Go.
Others would point out that some general guidelines are useful for beginners;
or, when we reach a new "beginning phase," which happens repeatedly in our growth curve.
That's all good.
But the bottom line is this: in Go, the exceptions are the rule.
Corollary: the first (and only) rule in Go -- There is no rule. (Same in the martial arts.
)
I'm sure Bill (and others) will have their brilliant replies, other ideas. This is my take, from my own experience,
and from watching the teachings and experiences of many other people.
you kicked at E3 as in your variation, and W replied with the normal extend F4 in your variation,
F4 exchange is good for B, bad for W.
was probably strange. ) After that, in your variation, 
at E6 feels a little slow. Tenuki instead.
did you consider K3/K4 area to attack W's only weak group ?
big but not urgent (F17 and E14 directions are miai -- you cannot get both anyway): did you look at Q9/R9 area ?
this shape is not great: locally, C15 kosumi is better shape. Also, did you notice W has a weakness at Q9 ?
your comment was correct: this feels strange.
yes, reply locally. Did you consider the E7 hane (head of enemy stones) ?
unbearable.
did you consider E7 atari ?
.
stone (then again maybe there is a stronger response here). This allows black the opportunity to approach the upper right group and settle their stones with something like 
(this last move may be wrong - I'm winging it a bit).
straight out is that white has to respond twice to