Will it benefit me to make a few of my counts dukes?
Yes, you will want to promote vassals eventually... If you are a duke, you can hold as many duchies as you want without penalties, but eventually your effective amount of vassals will be higher than your vassal limit, meaning that when you die, you will have a chance to loose some of those vassals. Becoming king and emperor increase your vassal limit, but you will get opinion malus for holding more than two duchies, so delaying becoming king is not necessarily a bad idea.
So, once king/emperor, you will want to get rid of most of duchy titles. This is also important to keep your vassal limit lower. Also, fewer stronger vassals are easier to keep in check than many weak vassals, because you can only placate a limited amount of vassals with council membership/marriage/opinion boosts (gifts, titulary titles, chancelor, carousing...). Eventually you will give king titles too, even if there is no limit to the amount of titles you can hold; just for easier vassal management and lower vassal limit (at some point you might want to have give multiple duke titles or king titles to the same ruler to keep vassal limit higher than your amount of vassals).
In some situations, you will probably want to keep BELOW your demesne limit; for exemple if you already fully own two full duchies yourself, you won't want to own a county in another duke's duchy or he will hate you for that - unless you're strong enough for his opinion to be irrelevant. You can own one county out of a two duchy title for exemple. In theory you can hold castle baronies in your counties but it is impractical: your demesne limit will vary depending on your rulers, so you will need to drop them or revoke to get them back. Later on, you will also need to decentralize your realm to increase your vassal limit at the expense of your own demesne limit, so owning baronies will become less and less an option. You do benefit a lot from extra levies in your capital county though, so it is still a valid strategy even if just temporary.
Landing your sons is certainly a good idea; this is a way to virtually increase your demesne limit, especially in gavelkind succession types; you can only hold a limited amount of titles that then get distributed among your heirs. However, if you starting granting titles before you die, you can effectively distribute so much that your future heir will inherit enough titles to end up at his demesne limit, or even higher. Beware though, as it happens that your heir die before you, so keep the best lands yourself to make sure your real heir will inherit it eventually.