there are engines and then there are good engines.
i personally have never had to seal an engine and have never had any issues.
however that does not mean that all enegines are the same.
when it comes to sealing an engine i first wonder how long the engine sat in a warehouse and then at the shop before it was actually purchased. the longer they sit the better chance the seals have to shrink or dry rot and crack, causing air leaks.
personally i by new ,usually from www.towerhobbies.com
where everything is stored in a climate controled environment preventing the break down/shrinkage/dry rot of seals.
even then when i get them i don't seal with the red high temp RTV that most recomend , i use green slime from team associated so when i have to disassemble for inspection i don't have the giant hassle of cleaning off the RTV.
green slime keeps the seals pliable and prevents shrinking and dry rot.
and has kept me from having issues such as air leaks.
i do this to new engines as well as my older engines.
if i do find that an engine has shrunken seals or cracked seals ,i spend that very small amout of cash and buy new seals to prevent further head aches.
here is a link for green slime.