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Corey R. ShanusRestoration (as opposed to mere surface cleaning) for memorabilia is completely different than restoration for baseball cards. For the latter such treatment is taboo; for memorabilia, for most collectors, it not only is acceptable but also recommended.
As has been noted, if an item is very brittle, appropriate restoration will preserve its longevity. For example, turn-of-the century paper is known to have a high acid content. There have been some mammoth plate and imperial cabinet photographs I own with deteriorating mounts that I have been advised to have rebacked. Publications often have detached pages with decrepit bindings. Rebinding in such instances is a common and accepted practice. In the case of hand-colored lithographs or chormolithographic advertising pieces, repairs and reinforcement to mounts/backings, and in some cases incoloring, is common practice.
However, whatever one decides to do, should he/she ever decide to sell the item, appropriate disclosure is a must.
In regard to value, while of course all things being equal, of two pieces with substantially identical outward appearance, the one with no (or the lesser) restoration will sell for a premium. But it has been my experience that often the extent of this premium is not that large. Desirable 19th century memorabilia is just so rare and in almost all cases in such need of repair that restoration has become very commonplace.