With a 3.3V Arduino, you don't need level shifters.
Alternatively, if you are very careful, you can use I2C between a 5V and 3.3V device without level shifters. This is because the I/O lines are not actively driven HIGH. However, you must make certain that the required pullup resistors on SDA and SCL are connected to 3.3V and not to 5V.
With SPI, the I/O lines are driven to 5V by the 5V device, which is why you can destroy 3.3V devices by trying to use SPI from a 5V device without level shifters.