Topic: straight cut vs. fades

Any recommendations about straight cuts vs. cross fades? Here's the situation. 51 slides in a 3:43 minute audio slide show. About 4 seconds per slide, some more, some less. When I used a 1 sec for cross fades I got ghosted images ( I could see the last slide through the next slide) in video converted form. Did not get ghosted images is test mode. Next I used the straight cut between slides and that solved the problem, at least in the video converted form. Thus, my question. Any rule of thumb about time length for cross fades? Thanks, John Runnette