I do not represent apple, so of course take everything I say with a grain of salt.
With that said, charging speed has a few variables to consider on wear:
1) Quality of the charging device generally -- the cheapo chargers you buy on eBay might have poor regulation and have an unstable voltage, a ripple current, or other noise -- this has a high likelyhood of wearing out your devices. There are some good 3rd party devices, but you should stick to high quality 3rd parties such as Anker, and not just random no-names from eBay, or random counterfeits from Amazon.
2) The number one thing that wears out batteries is high heat, so having your laptop charge while it is sitting on a soft surface, or storing it in a parked car, etc is going to wear it out faster than anything else.
3) Faster charging will lead to higher heat, so in that respect, a 60W apple geniune charger puts less load on the battery
4) Slower charging results in more runtime on the charging circuits in the laptop, and in the charger brick -- so a 60W charger will have slightly more wear than an 87W charger, as will the charging circuits in the laptop if you use a 60W (or lower) charger. If these things are made properly, this shouldn't matter as these things are designed to be solid state. In that respect, I am still using my original iPad charger which I bought with my iPad 1 without any problems
5) Using a lower power charger may result in the battery draining as you are using it, depending on workload. From my experience, a 12W charger will result in slight draining while using moderately (web browsing, email, Terminal.app/ssh, screen at half brightness), which means I could get >24 hours of use in this respect. I use one of these in my kitchen to charge phones, and also to use my laptop without draining the battery on the kitchen bar while I eat breakfast (
https://www.amazon.com/Leviton-T5632-W-Charger-Resistant-Receptacle/dp/B00J3PMU4C ) (
https://www.amazon.com/gp/product/B00S8GU2OC/ref=oh_aui_search_detailpage?ie=UTF8&psc=1)
6) Bad USB-C cables can fry your laptop, make sure to use one that is "Benson Approved" (
http://bensonapproved.com/ )
7) Similarly, I use one of these in the car which puts out 15W on the USB-C port, and results in neither charging nor discharging while under moderate use (slightly brighter than half, web browsing, email, Terminal.app/ssh)
8) I don't own a 29 Watt or 60 Watt apple charger, but I would expect both of them to charge the laptop under moderate or even moderately heavy use, and to discharge while under very heavy use.
