When using LEDs in TTL circuits,
should I use a transistor (or something similar) to drive LEDs
so that the LED does not affect the rest of the circuit too much?
Henry

Look at the Isink (or Isource) spec on the datasheet for the device.
If it will sink[1] or (less likely) supply the current that you
require,
the answer is No.

[1] The connection is from Vcc, thru the LED,
thru the dropping resistor,
thru the lower half of the totem-pole output to ground.
(You will have to adjust your logic accordingly.)