My $.02 ,
You should rarely get over the rear of the bike on a jump unless there is a kicker that tends to kick the rear end up. Small take off ramps, where the front is already off while the rear is just starting to compress will kick the rear up, You NEED to be on the throttle to prevent this.
Normal jumps should be jumped in a central/neutral body position. This allow you to compensate when the attitude of the bike is unexpected. If you consistently jump nose low, you are probably choping the throttle (or else sitting on the jump face, another no no unless you are prepared and on the throttle).
Jumping with your weight back is dangerous in my opinion. You have a higher chance of looping out if this is done BEFORE you leave the jump. Shifting your weight around should really be done after you are in the air to compensate for problems.
You will notice a lot of fast guys seem to be over the back of the bike. This is ussually because they are going faster than needed to clear a jump and are braking on the jump face. They shift weight back since they are not jumping under power. This is an advanced technique (IMO) and should only be done after you become comfortable jumping (and are comfortable making mid-flight corrections).