No...not always as you have discovered. But some degree of rake is the norm. It really depends on the time period and type of ship. Most good model kits will have plans that should show the rake of the masts for that ship. Then you can fashion an angle measure from a piece if scrap wood or card stock to fix the rake. Oh, for many multiple mast ships the rake of each mast can be different as well. I recently did a model of a three masted ship circa 1780s and all three masts had a noticeably different rake.