Neither oversizing or undersizing helps. The manual J method provides an excellent way to determine proper size. 90% units are as efficient at sea level as they are 6000 ft. What is different is you get a rerating on high altitude firing. So a furnace that may be rated 100,000 BTU could be 5 to 8% less like 92,000 to 95,000 depending on the manufacturer. This firing rate may be where the oversizing reference is made. This sizing is done to compensate for the altitude. Still it is a 90% furnace. The major manufacturers have high altitude kits to achieve the best performance. Colorado is also a "green" state so there maybe incentives offered for high efficiency equipment.