How to perform cross-validation using svmtrain polynomial function?

I am using SVM polynomial kernel(Statistics tool box) with order ranging from 3 to 9 (Say). When we perform cross validation with rbf function, we intend to determine sigma and C values. With the best sigma and cost factor value the trained network is tested on the test set. Do we determine cost function and sigma as well while performing cross-validation using SVM polynomial kernel? The commands that I intend to use are

structtr = svmtrain(z1,trainoutput,'Kernel_Function','Polynomial','Polyorder',i,'showplot',false);

grouparousallat = svmclassify(structtr,w1,'showplot',false);