When the amsmath package is used, ellipsis dots should normally be typed as dots. Placement (on the baseline or centered) is determined by whatever follows the dots. If the next thing is a plus sign or other binary symbol, the dots will be centered; if it's any other kind of symbol, they will be on the baseline.
If the dots fall at the end of a math formula, the next thing is
something like end or |\)| or |
A_1,A_2,,
the regional sum A_1+A_2+,
the orthogonal product A_1A_2
,
and the infinite integral
Specifying dots this way, in terms of their meaning rather than in terms of their visual placement, is in keeping with the general philosophy of / and makes documents more easily adaptable to different conventions.