For predominantly "buy and hold", generally, the earlier the better. I have some money from each paycheck automatically diverted to a savings account, so each January I have already saved up the full legal limit for contributions and I just make one contribution for the new tax year.

Some people contribute monthly or on a per paycheck basis to their Roth IRA--many mutual fund families don't charge a per-transaction fee, so if this is the easiest way to get the money into the Roth IRA, it could make sense.

If one is dealing with a discount broker, one would generally want to make sure that the transaction costs aren't more than about 2% of the amount being invested. Someone else may have to answer about the best approach with a discount broker--I am strictly in funds at the fund families, no transaction costs, but I, like all mutual fund investors, get hit with an expense ratio.