One of the questions someone asked me recently.
It depends on how much you weigh, what thickness wetsuit, and what you are doing. Sometimes if I'm diving shallow for halibut I'll add extra weight so I'm not bobbing all over. It also depends on where you want to be negatively bouyant at, it's always safer to be less negative at shallower depths because when you are digging out a fish at 60' you don't want to be so negative that you have a hard time kicking up. You can do a rough check in a pool but you might have to add a bit of weight later because of the salinity vs freshwater.
I use a harness because I like that weight to be on the center of my back and not my waist, but it's way harder to ditch, it is also easier for stuff to hang up on if you aren't careful, if you are one of those crazy mofos that climb deep into caves for lobsters, a harness is not for you. If you plan on using a freedive recovery vest, a weight harness is not for you.
I use it because one time I had a really thick, warm spetton suit, it was 7 mill top and full 7mm farmer johns, it was probably the warmest suit I ever owned and when it was new I floated so bad I was carrying a ton of weight. I was shore diving and that walk killed me, not to mention doing repetitive dives for halibut was killing my back, I switched out for the harness and only use a regular weight belt when I'm diving my 5mm suits.
You want smaller weights rather than a couple of gigantic 5 lbers, a series of 2 or 3 pounders with belt keepers to keep them from clanking around work out great and will be less painful on your hips. You can pick up trim weights at the dive shops that have bungee attachments that you can quickly add and remove depending on what you need and that way you will have a set weight belt.
I've torn a billion rubber belts. A rubber weight belt stretches so it's not always falling off at depth once you cinch it right. A wise scrawny boy (Dam) told me later, "dude you want to get the rob allen belt, rob allen weight belts last forever". He was right, I picked one up from Mori and that belt has not ripped at the buckle holes and is probably six years old now. The others I had were copies of the Marseilles style as well and even though they looked the same they would tear at the buckle hole and last a year or less (I'm hard on my gear and do a lot of shoredives).