Kayak trim is all about having the boat rest in a neutral balanced position in flat water.  If the stern is heavy, smaller holes and ledges will be more grabby and often cause back enders when you least expect it.  If the bow is too heavy, you will be pushing water all the time or need to lean back a bit to lift the bow placing you in a more unstable boating position.  Side to side balance is also important.  Although you can guess whether you are bow or stern heavy, it is far easier to have someone else observe your trim when you are in the water.  Most kayaks start with their seats in a reasonably balanced position.  We then add gear like heavy throw ropes, pin kits, first-aid & repair, lunch, water, etc.  Since behind the seat is easier to reach, we typically stuff all that gear in the back of the boat.  At least try to balance the load spreading the weight evenly on each side.  If the stern of the boat is sinking, one approach is to move the seat up one notch at a time (the foot bulkhead will also need to be moved up as well).  Another neat approach is placing seldom used gear in a float bag in front of the foot bulkhead - perhaps the first-aid and repair kit is a good choice.  Consider spreading the load of group gear, one person carries the 70' heavy duty line, another carries the big first-aid kit, etc.  Many boats have ways to carry water bottles in front of the seat and water does weigh a fair amount.  Your goal is to be able to sit upright in flat water with a reasonably balanced boat.