if you're thinking of a whole work, like a sun square, there are times when it is better to start.
If you're thinking of daily meditations like viod, aura cleaning, aura of protection, chakra meditations, and so on.. Since they should be done daily, there is no such a need to time it to the planets.
If you want to do a "daily" type of meditation very intense for a few days, so you make a "work" with a "daily" type of meditations, you can time it, in fact you should. Like an intense AoP work for 10 days or something.
And obviusly a not a "daily" type of meditation like as I said any magic squares, or a room cleaning, or creating a thoughtform, or doing some rituals, and so on... You can time them, but inside this category there are works, or meditations, that you should time them, like a sun square, and also works, meditations that are not necesarry to time them, like a room cleaning, power rituals, and so on...
So it's a kind of complicated question, but very easy: if the ritual or work or the meditation says that you should time that, like you should start it at waning moon, well do what it says. If something does not say that you should start it at a specific date, then you don't have to, but you can, but it won't have so much benefits.