I am a small contractor and we do a lot of pond work and I would agree with Jason7858, Line the entire thing the first time because more than likely you will have to redo it, what happens is if the soil is not very dense the water will leak under the liner and then you have another mess and a lot of hard work wasted. If you are confident the soil is OK on the slopes I would at least make sure you dig a key trench around the liner and spread and compact bentonite after you back fill the key way. As far as the slopes we do have tough time once in a while making sure the soil doesn't slide down but like he said every pond is different. We have used bunker covers in the past, witch is silage bags just not in bag form, and have had very good luck with it. You can get it in lengths of up to 1500'l x 150"w and anything in between, plus it is very easy to work with compared to other liners, I need to note the quality is nothing like the actual pond liners but is a fraction of the cost and if care is taken the results will show. An example is we did a 4.5 acre pond and the liner cost was 17k compared to over 100k if we used the actual pond liner. We also use a spray adhesive to seal the joint and overlap by about 10'-15'. If you have any questions please let me know. Best of luck with everything