I tend to buy in bulk when something is available (for expensive components: double what I need for a build, for inexpensive components like resistors, diodes, zeners, bc5XX, KSP92/42 transistors etc I buy 100), because:
1. bulk discounts,
2. free shipping if you spend above a certain level,
3. useful to have spares so if you make a mistake in building it initially or in 10 or 20 years time you can still service and repair the amp...
4. you never know when the there will be more stock.
5. you can sometimes swap parts with other people
6, you never know when ON semiconductor will delete the component or take over the manufacturer (as they did with Toshiba) and then delete all the useful parts
7. component prices only go up especially when they are no longer manufactured
8. you are less reliant on ebay or other doggy sources of possibly fake components.
9, more choices if you need matched pairs, or need high hfe/Beta etc
I keep my inventory in an excel spreadsheet, so I know what I have and what I am getting low on. I also add to the sheet what build(s) each component is used in. Which helps with making decisions on quantities and buying priorities.
for example I have 24x 10m90s because they are used in the golden reference hv psu - which I use in every build I can and are used in the modern t2 psu, T2 amp, and blue hawaii...