At what angle do you fire your roundhouse kick during sparring [olympic style sparring though]? parallel to the ground/perpendicular to target [90]? 70? 45?

My club keeps telling us to keep body as forward as possible, twist hip in as much as possible to fire directly perpendicular to the target [90 deg], but it's kinda slow -.-

I'd love to help you with this but I bring that kick from everywhere.  I guess by the book 90 is ideal but I'll bring it at 30 with no chamber just to get under your guard..We are talking strictly body shots right.

yeah body shots. although i won't mind knowing about head shots too

With as little angle as practical, quicker and with the least body movement, telegraphs less

Sparring i do at anything between 5 and 45 degrees (got to get through that guard!) but in drill work somewhere between 75 and 90 degrees for correct form

I'm confused...Why would you keep your body as forward as possible? I have been taught that you must bend your knee pointing it toward the target(gets your leg there faster) releasing the kick once the body is turnedand fold the knee back right after you kick. It makes the kick faster. I don't know what angle, but I know we practice Hapkido and my kicks got faster to keep others from grabbing my leg and tossing me to the ground! LOL As far as keeping forward, I stay side-ways (or try to anyway)! And yes, you gotta use your hips, but it's more important to pivot the opposite foot in order to utilize momentum...We practice lead leg roundhouse kicks to strengthen those particular muscles that help you stay sideways to your target. If you face the front, don't you get kicked by front and side kicks more easily?

Problem with Olympic-style sparring is because, as speed is movement specific, most people are more adept with roundhouses[most common kicks] than front and side kicks, which means it will be easier to dodge front and side kicks [and because they're straight].

When we're in fighting stance we face 45deg[to prevent getting hit on the back], but for kicking, Keeping body forward while only twisting in hips and foot[not 170 deg like traditional roundhouse] is because 1. it takes a lot of time to continue the combo with the other leg if the opponent switches side, 2. telegraph less, experienced fighters whom I know "foretell" kicks by shoulder movement, 3. countertorque [check the other topic on roundhouse i made], 4. Say after the roundhouse, u drop ur leg infront, and bring it up for a frongleg roundhouse, it will still be going diagonally up from the side, instead of vertically and just using the power of the snap.

The most our body turns is 45 deg, basically, control over power. The only time we turn our bodies like in the traditional kick is when we need the distance [body less turned = shorter distance coverage], or we intend to follow up with a spin kick[including 360 n stuff] or axe kick etc, or there is a substantial target for us to "bounce back" or to stop our rotation, but problem with class is the things we kick are hand mitts.

And yes, even with body forward as much as possible, we twist out hips until our knees point towards the target [even at 90 deg, although i cant do it -.- higher grades in my class can do it so naturally]

Here are two interpretations I have found:

1-Use a 45* angle, and don't twist your entire body into it.  This way, you still hit (approximately) the same target area (i.e. abdomen) but are much quicker than a full-out turning your body into it with a 90* angle.

2-Use a 90* angle for more power.  The action of twisting the hips, waist, torso, and legs puts noticable more power than a 45* kick.

There's a lot of information here, but basically I agree with Stone Tiger. Remember, you don't get the point merely by making contact to the target. It has to be a resounding hit and you can really only do that with the 90 deg. option.

I bring the knee straight up meanwhile begin pivot and pull hips through while leveling out the kicking leg parallel to floor for the 90. Keeping the body forward is important for your balance and continuing to move toward your target on the finish. You have to finish the first kick before starting the 2nd to really have control of both. To do that effectively, you need to have the recoil from the first kick while still keeping body forward so you can plant in fighting stance for the next kick.

It's so hard to describe these techniques in writing without being able to show what you're talking about isn't it? 

If you are interested in power roundhouse kicks, there is a snippit in the November 2008 Taekwon Do Times.  In it, the author advocates using a Mui Thai style of attack...