Precautions necessary when removing toilet and sink for extended period of time?
If you remove the P-Trap (the S shaped pipe) from the drain of the sink, then yes, you need to plug that pipe up as well.  

The toilet drain has no trap - the toilet itself is the trap, and so it will need to be plugged regardless.

For the drain to the sink, its often easiest to simply affix a permanent cap on the drain pipe and then cut that off when ready to work again, but you need enough of a drain pipe to be able to cut a few inches off later and still work.

For the toilet, you can pick up a toilet drain plug which you can insert into the toilet drain and tighten to form a seal.