Piercing potatoes with a fork before cooking them is a common practice that helps steam and heat escape from the potato while it cooks, preventing it from bursting or exploding. This is especially important when cooking potatoes in a conventional oven or microwave. 

When using an electric roaster, the cooking process is slightly different. Electric roasters are designed to provide even cooking and better heat distribution, which can help reduce the likelihood of potatoes bursting. However, if you want to err on the side of caution, you can still pierce the potatoes with a fork before cooking them in the electric roaster. This will allow any built-up steam to escape and ensure that the potatoes cook evenly without any potential for bursting. 

So, while it might not be as critical in an electric roaster compared to other cooking methods, it's still a good practice to pierce the potatoes with a fork before cooking them. This can help ensure that your potatoes cook through evenly and without any issues.