The two problems I have with putting the Dagobah at the start:

1) When Luke was training on Dagobah during Empire he left when he found out Leia and Han were in danger - now he knows that Han is in even graver danger, so for me it's a blatant character contradiction for him to go back to Dagobah now instead of help rescue his friend

2) Presumably if Luke doesn't go to Dagobah after Tatooine he goes with the rest to the rebel briefing, so must have been there for the whole of that scene - but there isn't a single shot of Luke you could use to show that he's there listening to Ackbar and the rest