It depends upon what your goal is with rebalancing. If it is to maintain a risk profile, then daily would be best. Beyond that, it's really just personal preference; the real effect on your portfolio will be small if you do it once a month.
Consider this - if your equities have a bad month, down 10%, and your bonds go up 2% in the meantime, how much will that change a 70/30 asset allocation?
Assuming a total portfolio of 100k at the beginning of the month, you have 63k/30.6k at the end of the month, or an asset allocation of 67.3/32.7. Is the risk profile of 67.3/32.7 significantly different than 70/30? I submit that it isn't, and that your daily rebalancing didn't change a whole lot. If you continued to invest with your original asset allocation during the month, your deviation would be even lower.
Now, there is another possibility - though it is a controversial one - on a potential "rebalancing bonus." This is an attempt to capitalize on "reversion to the mean," in which specifically set or timed rebalancing triggers will force you to buy low/sell high, or so you hope. There are studies on this, which suggested that - in the past - triggers set around 15-20% relative for each asset class, checked with a certain frequency, maximized the rebalancing bonus. No guarantee that this will be most efficient in the future.
Retirement investing is a marathon.