When training during class and in patterns, proper performance of Taekwondo techniques is usually strictly enforced. The way Taekwondo is practiced depends on the association, school, and individual instructor. Some instructors teach strictly Taekwondo while others teach a base in Taekwondo with variations depending on what works for the individual student.
While sparring, you may use any type of block, punch, or kick you choose as long as it is within the rules. However, this sometimes makes it more difficult for you to excel in Taekwondo. For example, I fight with open hands and use many open hand blocks. However, when I punch, my hand instantly changes into a tight fist. I have training this way for years so I am able to stay loose and relaxed while moving but I can instantly tighten everything for a powerful attack. This causes problems since when I perform patterns I am constantly criticized for relaxing my fists during a transition and then tightening them for the technique.
Sometimes a karate background may be advantageous. Taekwondo uses hand attacks but you may find that TKD fighters are surprised by your quick, powerful karate hand attacks.