I think, although I haven't tested this directly, that mulching with woodchips on the soil surface has minimal affect on soil acidity. Basically, as most wood chips breakdown they tend to drift towards neutral (as does
compost for that matter).
My reasoning is that the potential risks (slight possible increase in soil acidity) is massively outweighed by the improvements in soil structure, fertility, moisture retention.
Would I prefer hardwood? Probably. Will I take any and all chips I can get? Absolutely.