This question probably be better in the elements forum. However if you crop or not depends on how much control you want for aligning the picture on the screen. In general the picture will fill the screen, But there will be times when you want to make sure a persons face is on the left 1/3 of the screen for example. When you crop or scale a picture is best to make sure it is the same size as the screen so there are no surprises.
Technically the apps generating the slideshow can determine if the full size photo will be a fill or fit, which is why its a good idea to scale or crop the photo's first. Fit means all the image will be in view and can result in bars either top/bottom or left/right. Whereas fill will scale a photo until the entire picture meets all four edges of the screen and can leave part of the image out of view.
As for what size to crop or scale to, that is depentant on the TV. Older TV's (pre-flat screen) were 720x480 or SD. Newer TV's are 1280x720 or 1440x1080.
Since you are burning to DVD and not Blu-ray you are limited to 720x480. Widescreen means the size of the pixel or the ratio I should say is no longer square which gives you a wide screen even though the resolution is the same.
It's reasuring that the spec for projected images submitted for Photographic Society, and International Exhibitions and Salons, is at last reflecting advances in hardware. SVGA (1024x768 pixels) has been the standard for some years now, but just this year we are seeing requests for 1400 x 1050 and I have even seen submition rules quoting 1600 x 1200 pixels. I am not sure how the latter would be projected, because unless I am out of touch (which is highly likely) digital projectors only go up to 1080p which is 1920 x 1080 pixels.
Hmmm. I decided to check before posting, and there are indeed 1600 x 1200 projectors (UXGA) available. Which means a lot of Camera Clubs are going to be looking at some serious outlay of funds.