When sub-ohm vaping, it is generally advised to use lower nicotine strengths due to the higher vapor production and increased absorption rate. Nicotine strengths between 0mg and 6mg are commonly used for sub-ohm vaping. The specific nicotine level depends on personal preference and vaping habits. It's important to note that sub-ohm vaping delivers nicotine more efficiently, so it's recommended to start with a lower nicotine concentration and adjust based on individual needs and preferences.