i recently did my OME 891 install and decided to not go with the cones.
although you can if you want because mainly it will give you a slightly higher lift, maybe 3/8"s or so. also some say that it will help with non metal to metal contact between the coil and the body.
but i chose to not install becaues 3/8" to me isnt much and i havent had any issues with the coil contact when it was on my 97 since they didnt come with cones anyway.
also the OME 891 probably wont even get close to using those cones for their purpose anyway.
MAIN reason i chose not to install was because i noticed there could be a possbility under full flex that the cones would limit my compression and that outweighs the benefits listed above for me.
i rather have more flex up and down then to have 3/8"~ possible lift.