Rewards in Celestia have to be claimed manually. You can do so via your wallet or the CLI. Note that your rewards sit idle and do not earn any rewards (i.e. no compounding effect).Once claimed you can choose to also stake them or simply keep them on your account.
Once you have staked your TIAs, you are contributing to the network's security. In return, you will receive rewards in the form of newly minted TIAs. Conversely, if you do not participate in staking, your assets will get diluted over time.
The Celestia network states that validators are subject to punishment in case of malpractice. This is referred to as "slashing" and can occur in the wake of two events: the validator signing two blocks at the same height ("double signing") or the validator being offline for roughly half a day ("uptime"). The penalty for double signing is 5% of the total stake. The penalty for breaching "uptime" is significantly lower at roughly 0.01%. Penalties are imposed on both, validators as well as delegators.
Therefore, we advice you to carefully choose your validator. Please note, that the costs for running a secure and professional validator need to be covered by the commission rate. Hence, it is worthwhile to accept a certain amount of commission in order to secure your profits and minimize the risk of punishment. In addition, we have our own capital on the line in order to fully align our own interest with that of our customers.
Further risks include: key/asset mismanagement by the enduser resulting in loss of funds; protocol errors; or attacks against the network.
The validator you delegate your tokens to is not able to access your funds at any point in time since you are only delegating your staking rights. Your TIAs always stay in your wallet and you may re-delegate them to another validator at any time.
Unfortunately we cannot answer this question in a general manner as tax regulation differs among legislations. Nevertheless, we advice you to always track your staking operations so you can provide a detailed history of your staking rewards.