^^^^ What those guys said!^^^^
I got my own routine down... First, I NEVER try to get by with a china made bearing. If I get tricked into buying them, I either toss them back at the dealer or throw them all in the trash. Buy the brand name and buy the proper RPM/load rated bearings... no tricks or china scamming that way.
I remove seals and repack every bearing before I use it, whether it is a new or used bearing. I use synthetic grease to repack after washing out all of the Yak Fat they put in them. You can use whatever grease you think is appropriate.
If you take rubber seals off from them, do so by pulling the seal out of the groove on the outside diameter of the seal, NOT the inside diameter.... the inner part of the rubber seals have a delicate lip on them that WILL be damaged/ruined if you mess with them. Poke a narrow flat blade screwdriver (micro screwdriver sets usually have one or two with the perfect sized blade) into the top edge of the rubber seal groove in the bearing and gently get it started out... then slide the blade around that outer diameter until the seal pops off. You can do it without bending the seal, but if you do bend one on occasion simply flatten it back down using your fingers.
After cleaning with solvent and blowing them dry, I pack them completely full with grease, then with the bearing held between a thumb and finger, rotate the bearing so that my thumb and finger skims out the excess grease to allow for some expansion space. It doesn't require much air space... so don't empty out the bearing fussing over how much should be in it. Then put the rubber seals back on by using your thumb to gradually rotate/press it back down around the outer diameter ONLY, until its snapped in all the way around. You'll never touch the delicate inner diameter lipped part of the seal using this method. Chances that you'll damage the seals are very near zero...
If bearings have non-removable steel shielded seal on them, I drill a tiny hole in each side (180 degrees apart from the other) and force fresh grease in using an injector needle and grease gun, alternating holes and rotating the bearing until all the Yak Fat is out and my own grease is coming out pure. Then hit one of the holes with a bit of air pressure to give it some small amount of expansion space, then alcohol wash the shields and seal those drilled holes up with 3M Weatherstrip Adhesive or JB-Weld epoxy, depending on where its going to be reinstalled at.
I can literally go from getting a few weeks/months/years of useful life out of a potential high quality bearing, to getting a few decades out of them after a good cleaning/repacking. Anyone can... just a choice.