In general, like with nature, most of the trees are "sown" during autumn. This allows the seeds to go through the natural process of Stratification - in which the seeds are going through the cold season of winter, getting ready and preparing to sprout during spring. When you sow and grow indoors, you can basically manipulate the natural process and control the different climate and surrounding of the plants. You can sow your seeds at any time of the year as long as you carefully follow the instructions on how to keep them in the right temperatures and surroundings.  If you'd like, you may store the seed packets in a cold place for 1-2 weeks prior to sowing them, to mimic the cold season which could increase the germination rate. It is not crucial though as most of the seeds will germinate quite easily even without stratification.