If any of your transactions reference a date prior to the effective date on the dimension, this could get somewhat confusing.

For an initial load, I generally use a "placeholder" date of "19000101". That way it's obvious that the date doesn't have any meaning other than ensuring that the dimension entry is valid prior to any facts referencing it.

Alternatively, you could work out what the first date each dimension entry is referenced, and use those as the effective dates. That can be a lot of work (depending on the size of your fact tables) though, so unless you have a business case for why that would be necessary, I would suggest that a placeholder would be easier.