Should I answer an EiC email that communicates a revise decision and sends me the reviews, saying that I will? Or just revise an upload the revised manuscript when it is ready?
Just submit the revisions when ready, according to whatever the procedures for that particular journal are. No need to bother the editor with an extra message that doesn't require any action from them.
Often, journals request revisions within some time interval; if you expect to need more time, for example if reviewer requests will require additional experiments, the journal will probably ask that you request an extension. Otherwise, no contact is necessary unless asked for until you send revisions.
I, myself, would probably want to send a "thank you" email in return and mention that I'm working on the revision. The editor then knows, at least, that you got the revision request and that it didn't wind up in junk-mail limbo.
Some might say that it is a mistake to "bother" a busy editor. But, if the volume of work of an editor is so large as to make this a problem, then I think that editor probably has an assistant who filters the mail in any case.
It might be useful information to them or not, but the thank you part seems to me to be appropriate. If a reply were essential, the mail from the EiC would have requested it, however.