For most beers you don't want any alkalinity, and for those you do want some alkalinity it's a modest amount. The starting water pH has very little affect in these cases.
I'm not sure what "you don't want any alkalinity" means--you certainly don't want a sparge water that's nonalkaline (under pH 7) any more than you want a mash pH over 7, so I'm not sure I follow you there.
Starting water pH definitely has an effect on the mash pH. The apparently really common idea here that it doesn't is a myth caused by oversimplifying some reasonable advice: Don't worry as much about strike water pH as mash pH, because mash pH is where your final target is and there are other factors to consider. I don't want to beat this horse any more but I am going to sum up briefly because I usually talk too much and bury my point.
There are three main variables to mash pH as I see it:
- Grist roast (roastier = more acidic)
- Grist quantity (higher gravity = somewhat more acidic, but it is self-limiting)
- Water pH
To the extent that the first two two stay the same (like within the confines of one recipe), your water pH is the main thing determining your mash pH.
All this said, if you don't seem to have a problem making session blondes or light lagers that don't taste like tea bags, you don't have an alkalinity problem, and you don't need adjustment (but by all means experiment away). Optimizing pH has a positive effect on sugar extraction too but I think efficiency is pretty unimportant at homebrew scales as long as you get it consistent.