I am a software guy, so this is based on how I understand the protocols, etc. It may or may not have any basis in reality.
For i2c, you need something that is bi-directional, but generally your wire length is pretty short. I2c is a slow protocol, so you do not needing fast transitions, and most of the level converters will work at base speed. If you go up to the faster speeds, then more attention needs to be paid to the details.
For WS2811/WS2812's (i.e. Adafruit neopixels), the protocol is all one directional, and there is no clock pulse, so every bit must be delivered to a exact timing window. This is one of the classic articles looking into level conversions with WS2812's:
http://happyinmotion.com/?p=1247. The length is the entire length of the LED string, so you start seeing more issues due to wire length, etc. In WS2812's the data flows in a single direction, so you can use the 74HCT245's (which are fast, but single direction).
For small numbers of neopixels (32 or so), I've used the DSS circuits TXS-0102 (
http://dsscircuits.com/sale/product/dssc0105) and the Pololu logic level converter (
https://www.pololu.com/product/2595). Pololu suggest I add 10k pullups on both side (
http://forum.pololu.com/viewtopic.php?f=3&t=8858) if needed. For the small set of neopixels I use, both with the pull-ups and without seem to work fine. Note, this was just looking at the pattern, it did not involve detailed measurements.
Of course with the LC, if I'm only using one stream of LEDs, the logic conversion in the LC works fine.
The Adafruit neopixel guidelines (
https://learn.adafruit.com/adafruit-neopixel-uberguide) say to have a 300-500 Ohm resistor in front of the first neopixel LED, which IIRC, is to prevent ringing in long neopixel strings, and to have a large capacitor between power & ground to buffer power spikes.
If you are driving WS2812's at a distance, I saw this article about using RS-422/RS-485 drivers to send the data over long distances:
http://www.teknynja.com/2014/02/driving-ws2812neopixels-rgb-leds-over.html
Whether you can use 74HCT245's for SPI probably depends on whether the communication is duplex or simplex. If it is duplex, you will probably be limited by the level converter speed.