For various reasons besides punctuation and case, street names in addresses can differ from the street names that are usable for wayfinding. For example, the houses along this street use “Delhi Road” and “Delhi Pike” interchangeably in addresses, while the signs leave the situation quite ambiguous. Another common case involves a road that carries a numbered route; the standard address may read “US Highway 44 W” while the signs refer to “U.S. Rte. 44”, “U.S. 44”, or simply “Hwy. 44”.
I think addr:street and the street’s name should be accurate with respect to each feature. Whenever there’s a mismatch, I tag the street’s alt_name so that a geocoder will hopefully match the street and address anyways.
The MUTCD preference for mixed case is relatively new; we’re still in a national period of transition from uppercase.
Street name signs are an important factor but not necessarily the final word on the subject. For one thing, some jurisdictions drop the suffix from all their roads’ street name signs as a matter of policy, even if they acknowledge the suffix in writing and speech. (Maps do this too to squeeze more labels in tight spaces.)
The MUTCD also prohibits periods, apostrophes, and hyphens from guide signs, regardless of the road’s actual name. Thus, John F. Kennedy’s name is signposted as “John F Kennedy”. But whenever an abbreviation is warranted, retaining the period is useful to data consumers, particularly those that send the street name to a text-to-speech engine. This is more relevant to name than addr:street, but geocoders do benefit from exact matches between the two keys, and the USPS has very robust systems to ensure that periods in addresses do not delay the delivery of snail mail.