Why not use 12v strips?
Because 12V strips also need an steady voltage and a 12V battery does not give that. I do not see any reason to use 12V strips vs 5V. But i'm not 100% sure off course.
I'm going to need an volt regulator anyway, i think?
12V LED strip is more efficient, than 5V strip.
12V LED strip consists of multiple strings of three LEDs in series, with a series resistor. Each LED drops about 3V, giving 9V in total going to the LEDs and 3V across the series resistor. 9/12 = 75% of the electrical power going to 12V LED strip, is delivered the LEDs.
5V LED strip consists of a single resistor, per 3V LED, 3/5 = 60% of the electrical power going to the LED strip is delivered to the LEDs.
Another advantage of using higher voltage LED strip is, the current is much lower, so there's less voltage drop over the length of the strip and cables. With a very long LED strip, the LEDs further away from the power connector, will be noticeably dimmer, as voltage is dropped over the copper tracks in the strip.
If you want to use a linear regulator, use 12V strip and a 12V low dropout regulator. The battery voltage should always be 12.5V, except when the engine is being started, so it will output a steady 12V, most of the time.
I would use a boost converter and 24V LED strip, which consists of strings of six LEDs, per series resistor and has a similar efficiency to 12V strip, but the higher voltage means a lower current draw.
The only advantage 5V LED strip has is, it can be cut every LED. 12V and 24V strip needs to be cut every 3, or 6 LEDs.