personally, i bitset every disc i burn to dvd-rom, but there's technically one possible downside to doing so. it might trick your pc dvd drive into reading the disc as fast as a pressed dvd-rom, which is a problem if: a) your pc dvd drive is a poor reader, b) if the burn quality of the disc is low, or c) both. this is only a problem if you're performing some type of high-speed read operation (such as ripping the contents of the disc) and the drive tries to read the disc too fast, in which case the drive must slow down. the only downside to this is possible performance loss.
conversely, this is an upside for very much the same reason. if your pc dvd drive reads recordable discs more slowly than pressed dvds and it can be tricked into thinking that booktyped dvd+rs are dvd-roms, then bitsetting to dvd-rom would net you a performance increase if the drive can sustain the higher read speed.