If they plan to claim, it is best on day 1. It really all depends though on whether or not someone is likely to be copped or not. If you have a tendency to be copped, then yes, claim D1, if not, well, we've no HC table.

Here's the thing, if we mass claim on say, day 4, normally a mafia member might notice on the HC table he'd been HC'd guilty, and consider a miller claim, especially if the HCing player had listed innocent for others they'd HC'd, otherwise you're moving yourself from prob-town or neutral town to prob-scum. We won't have an HC table, and we are likely to lynch miller claim, personally, so long as a miller claim takes place before investigative reveals, I'll consider it as valid as D1, keeping in mind of course that I strongly favor lynching miller claims as a general policy. I would rather people just claimed now, but I don't really expect people to, so if they aren't going to, I'd rather they were aware of other options. It goes without saying, embedding miller comments in day 1 messages for future reveal makes such claims more believable.

Note by the way that on my proposed Day 3/4 massclaim strategy calls for vanillas and passive action roles to claim first, this is primarily to prevent heightened probabilities of a scummer false-claiming miller and similiar.