Ideally no, that tends to cause balance problems. Best place for the heavy stuff is around the centre in height but close in to your back. I was trying to find a link for you on the work done by nz guy (aarn tate) on rucksack/bodypack ergonmics,if you get a chance google search aarn packs where he has a good explanation of this.
General guide to packing a ruck well is to have something very light right at the bottom eg your sleeping bag/spare clothes. Heaviest stuff then in next layer but with the densest weight close in to your back, then medium weight above that--in practical terms thats where your shelter system/foulies go.
One thing to check is that the ruck is of the right length,in my experience lots of people have packs that are the wrong size or shape for them--usually too long.
Getting used to carying load does take time and training,i am lucky in that i have been carrying rucksacks since my teens..even so if i am preparing for a trip i will train with increasing distance and load, also nowadays tend to 'shock load' with much higher weight occiasonally .