With a simple resistor circuit, and with the variation in the voltage on the LEDs and the battery, you would not know what LED current you would get with much accuracy. If you put typical numbers into an LED current calculator like this:-
https://www.mtrak.co.uk/led_calculator.html you will find that the worst case current could be far from what you expect.
If you allow ±10% for the battery, and ±0.1 V for the LED, then if you don't mind the current being anywhere between 10 mA and 28 mA, then the 150 Ω resistors will work.
With a boost converter and a 5% current sense resistor, all the LEDs will have the same current, the current will be within ±15% of what you want, the efficiency will be better, and the current will be constant over the whole life of the battery.
If you are using a PP3 battery, the voltage will fall to 8 V fairly quickly, and you need to be able to use the battery down to around 6 V if you are to get the best use out of it, so with resistors the LED current will be far less than what you want for most of the life of the battery.