I have had many leaks over the years fris but all from bad corks,
we try not to boil most modern corks , because most are made from cork particals rather than the better quality solid corks you can get some times,the boiling does them no good at all and can start them off to become brittel, we would normaly put the corks to be used in a meta sulf solution sorry metabisulfate solution of say one tea spoon or one camden tablet in a litre of warm water and then we put a plate on top to keep the submerged, i normal soak corks for around ten miniutes max some people just dip and some leave them for an hours horses and courses ,
I am talking about a proper wine bottle cork that will deffinatly need a corking inserter rather than corks that have a plastic stop on the top or tappered corks, so if your using stoppers rather than corks fris is spot on yes the bottles have to go upright, the main reason we lie bottles down rather than stand up is to keep the fluids against the cork, this prevents them from drying out which also causes leaks and can sometimes cause the liquir to go off as it were, if you had a wine that goes off we would normal call it ozidised or vinigar ha ha ha,
for some of the most helpfull people on pare with the beek forum try the "wines at home" forum they are a great bunch of people with far more and better knowldge than my 20 years of plonk making.
by the way is it just spirits your into or do you do beer and wine as well?