Many good properties in Lisbon are in a similar location to the Sheraton even the Four Seasons. Since the tourist sites are pretty spread out you are going to need to take cabs at some point anyway unless you are prepared to walk for a while. From the Tower of Belem it is possible walk back (plus use the Metro a bit) and cover all the core tourist sites as you do so (if you don't mind a long walk). Taxis are reasonably cheap and plentiful in Lisbon, I wouldn't worry about the location which is effectively on the edge of the business district.