I'm 5'6" with a 29" inseam and a background in road cycling. I'm interested in a used mountain bike with an 18" frame. When I stand flat-footed over the frame, I'm comfortably straddling the top tube, but there is basically no clearance. I can adjust the saddle to a proper height/angle to be comfortable while pedaling, and the top tube doesn't seem too long because I don't feel stretched out in the cockpit while holding the bars. I realize that I won't be in the saddle all the time while riding offroad (unlike road cycling where I'm in the saddle 99% of the time), so how important is standover height on a mountain bike? Should I pass on this one and keep looking for a smaller frame?