since you are using t-shirts that have probably been washed a number of times it would be a good idea to pre-wash (unless the t-shirts are new)
also kind of depends on what type of stablizer you used on the t=shirts? if you used a lightweight tricot fusable stablizer (the recommended stablizer for t=shirt quilts) and the t-shirts are worn/been washed numerous times- it would be a good idea to pre-wash the cotton fabrics so they do not shrink up and cause alot of puckering in your finished quilt