Ive just fitted Wickes MDF skirting in my conservatory & for the first time scribed the internal corners rather than cut both at 45 degrees (which I did elsewhere in the house). This was miles better & resulted in perfectly aligned corners, whereas before they'd always not meet at 45 degrees & need a squirt of flexible filler up the join (looks OK, but corners are then rounded rather than nice & sharp). External mitres are more difficult to line up perfectly in MDF as there is little give compared to timber. It sounds like a bodge, but I use plastic car body filler (isopen - the good stuff) up each side of the external corner then a bit of fine sandpaper to sand it into a nice sharp corner. Once painted, it looks ace.