We did Machu Picchu as a precruise visit last January, but we enjoyed the days we spent in Ariquipa and Cusco getting used to the elevation, before huffing and puffing to the top.
The bus ride is quite safe along narrow winding mountain roads. The alternative is a four day hike in along old Inca trails. There's a train ride from Cusco to the bus stop for the climb to the site.
The ruins are truly one of the world's wonders -- a must see if at all possible.