It basically changes one of the field on the disc's header. It has no effect on laser power whatsoever.
Some place like PC DVD-ROM where it judge maximum reading speed based on media, having DVD-ROM booktype might make maximum disc read speed to go higher. Sometimes, that is not a good thing to do, especially on poorly burn disc. Other than that there should be no issue. In fact, i set all my +R discs as DVD-ROM.:bigsmile: