If you are using friction shifting, you can use a narrower chain than is optimum for the number of cogs on your rear wheel. So you might get away with a 10 speed chain on an 8 speed cassette and possibly avoid chain rub. However, you might then have to deal with replacing your chain more often. It is all about tradeoffs. If you want to make gains in one area, you may lose something elsewhere. Having to trim your front derailleur isn't the end of the world, the more often you do it, the easier it becomes