Most slipcovers are meant to stay snug without grips. So, unless you end up with a wrong-sized cover, you would not require grips. However, old slipcovers may have lost their elasticity. In this case, you may invest in cover grips to help keep them more secure.  If your slipcover comes in separate pieces, you would not need inserts. But, single pieces of fabric covers are usually sold with foam inserts. This helps maintain the shape of the sofa. Read the details of the slipcover you buy carefully before opting for added accessories.