Start low and end high would probably mean that the situation changed. Nobody starts low and expects to raise the tax rate later on, it's just a decision to go for a tech at 50 turns that later is changed due to something unforseen.
One example that happens fairly often in my games is starting out (after Pottery) on Writing or Alphabet at 50 turns. Then I find out that I'm all alone on an island, so boost research to get off the island ASAP. There are other times I find neighbors, and instead use the cash I've saved up to get in the tech trading loop, and just finish out the 50 turns.
I normally run a very high unit/city ratio early on (mostly Workers), and high luxury rate, so the cash I save up while researching low often helps me catch back up to where I would be anyways. This often rears it's head in the Middle ages, where I need some cash ASAP for a trade or upgrade. So I run a high tax rate for a few turns then boost research again. This can happen at the beginning or end of a research cycle and often allows you to keep a 4 turn tech rate where you are losing cash 3 of the 4 turns. In such a case it's actually best to take the cash first, as either way 4 turns is the earliest you can get the tech. Requires a lot of calculating though.
Other times that researching low (or non-existant) early and then raising later can pay off is right before you will complete significant (and/or numerous) Libraries and Universities.
One example that happens fairly often in my games is starting out (after Pottery) on Writing or Alphabet at 50 turns. Then I find out that I'm all alone on an island, so boost research to get off the island ASAP. There are other times I find neighbors, and instead use the cash I've saved up to get in the tech trading loop, and just finish out the 50 turns.
I normally run a very high unit/city ratio early on (mostly Workers), and high luxury rate, so the cash I save up while researching low often helps me catch back up to where I would be anyways. This often rears it's head in the Middle ages, where I need some cash ASAP for a trade or upgrade. So I run a high tax rate for a few turns then boost research again. This can happen at the beginning or end of a research cycle and often allows you to keep a 4 turn tech rate where you are losing cash 3 of the 4 turns. In such a case it's actually best to take the cash first, as either way 4 turns is the earliest you can get the tech. Requires a lot of calculating though.
Other times that researching low (or non-existant) early and then raising later can pay off is right before you will complete significant (and/or numerous) Libraries and Universities.
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