Alot of new mowers use sealed bearings from the factory and don't even have greese zerks. I personally like to be able to grease the bearings. Both of my mowers i have drilled the spindles and added grease fittings to.
If your mower doesn't have greese fittings, definatally put sealed bearings in there. If it does have fittings, they usually only have one seal on each bearing that face outwards.