basically during the distribution phase VESTS are compounding at around 3% per week, so if you calculate the cost per Mv (million vests) and see a significant dip, then and only then would waiting make sense.
However, if you have to wait for 1 month for a 10% dip, it would have been better to have just paid 10% more and get 4 weeks of 3% gains (compounded)
If your timeframe of waiting is 3 months, then you need a 42% drop in price just to "breakeven" against this increasing ratio of steem_per_mvest
my advice is to just dollar cost average over a couple weeks, it wont get you the best price, nor the worst price. Alternatively to frontload it and become active poster/curator benefits from the increased rate of return from day 1.
RE: Investing long-term with Steem: Time as an overlooked parameter