I have only a little experience with perler beads, but from what I can tell there are different methods for ironing them, and you just have to choose which ones you like the most
1) light ironing
you will need to iron both sides if you use this method.
2) normal? ironing
you just iron it a little bit more and can choose to iron both sides or one
3) flat ironing
you iron the piece lightly then remove it from the peg board and iron the crap out of it till they look like square pixels, (dont' do this on your grandma's wood table or a plastic folding table etc..)
with this method, many people seem to like to only do one side and display the unironed side, but you could potentially iron both sides if you prefer.
to keep mine flat, I lay books over each section as I complete it (for a large one) then let it sit till it cools.
If you want to make a large sprite, but don't have enough peg boards, you can follow the masking tape method of creating a portion of it by filling up the board you have, and laying masking tape over it... like in this video:
https://www.youtube.com/watch?v=P5RjXH7AzTs
If you do this method, and want to go for the flat ironing, you'll have to remove the masking tape before the heavy duty ironing, or you'll have issues with the air expanding and blowing out some of your sprites.
there's probably more to know, but that's all I can tell ya! =)