You can better manage your data usage on YouTube by turning on limit mobile data usage. This will prevent you from streaming HD videos accidentally when not on Wi-Fi.
Limit Mobile Data Usage
Step 1: Tap the options button on the upper right.
Step 3: Under Settings, select General.
Step 2: Then select Settings.
Step 4: Turn on Limit mobile data usage by sliding the switch to the right.
This will prevent you from streaming HD videos accidentaly when not on Wi-Fi.