Do rewards accumulate, or do I need to withdraw each epoch?

Yes, your rewards do accumulate, but some rewards may have an expiry set, meaning that they will expire if you don't withdraw them until the expiry time. Always make sure to confirm how long it takes for your tokens to expire. This can be done by asking in the TosiDrop Telegram and Discord support channels.

Currently tokens of AADA Finance, Ardana, anetaBTC expire after 50 epochs of being available to claim.

Minswap tokens expire after 73 epochs of being available to claim.

AUDIT tokens must be claimed each epoch and do not accumulate.