I need a bike with a top tube of about 50.5 cm. To get a top tube of that length, I need to get a frame that is one or two sizes too small. That is, I would have to get a 49 cm. frame, whereas my inseam would indicate a 52 cm. frame. My LBS says that it doesn't matter that the frame is so small, but this strikes me as being counter-intuitive. Is the LBS right? Should I get the correct size frame and install a very short stem?