when you are in a widescreen project, export the frame and make sure the Pixel Aspect Ratio is set for 1.2. When viewed on your computer the image will looked squished that is becasue a computer monitors PAR is 1 ( What you are seeing is actually 3:2 not 4:3) when you reinport the frame premiere will read it as .9 PAR (4:3) (i wish this was less confusing) You have to interpret the footage manually and tell premiere the pixel aspect ratio on the file is 1.2 in order to view it without distortion. A program like photoshop will interpret the frame as 1 PAR to view it unsquished you must interpret the footage as 1.2 as well. If you take footage that is 1.2 and export it using square pixels the image will not be distorted rather it will letterbox a 3:2 image but the actual image area will be lower resolution. you can export undistorted frames without letterbox at a square pixel aspect ratio (so you wont have to interpret the footage in a photo editing program) by setting the resolution to be 720 by 405. Hope you can understand this (best way to understand what is going on is to play around with some footage of a person since it is more easily visible when a human face is distorted.)
16:9 project, file -----> export frame, PAR 1.0 (computer monitors PAR), resolution = 720x405, You probably want to deinterlace as well if there is a little motion in the video.
If you dont change the the resolution then you will get black bars on top and bottom
If you dont change the resolution or PAR and export it using a 1.2 PAR you will get a 3:2 squeshed image of your video