When To Raise
Most startups follow a similar trajectory as described by Paul Graham and Trevor Blackwell (of YCombinator fame) visualised below (screenshot from a Kiss Metrics presentation).
Caveat: If you've raised money for a startup before or founded something interesting previously, raising pre-seed before the "Initial Enthusiasm" curve starts to climb is entirely possible at a valuation you can sort of live with. Also, raising before and after is entirely possible for anyone - but take a wild guess as to how much equity you'll probably have to give up for the deal to close (and how desperate and unenthusiastic you are going to be when trying to pitch when you're in the "Trough of Sorrow").
Because after the initial rise in your relevant metrics you'll inevitably enter The Trough of Sorrow (where your real work will start) where your metrics will nigh flatline. You can imagine how interesting that is going to look to most investors (even if they know that this is the normal curve).
And keep in mind that the average (successful) startup needs 2-3 years to arrive at a Product-Market-Fit. Ideally, your Pre-Seed + Seed rounds should last long enough to get you there. Series A rounds and beyond are usually "Growth" or "Scale" rounds, ideally POST-Product-Market-Fit - when you've the stage where you can predictably repeat & scale sales and retain customers (b2b, b2c commerce) or predictably grow & engage (b2c social, "free to use" models).
I've added - in blue below - when you'd ideally be raising pre- and seed rounds.
