If you're playing hard (or normal), you can easily kill everything with your
knife (and even be at the top of the ranking at the end of the first wave).
This is particulary helpful if you're playing demo, or support, because of the low ammo pool from your T1. It's worth considering if you start as a medic (because you should not have to worry too much about the health of your teammate during the first wave, so it's a good way to gain some cash and get your T4 quicker. Especially because you're faster than all the other class except berserker).
It's not worth considering if you are playing firebug with fully stocked (lvl 5), berserker (obviously), commando if you are able to hit headshot (or using back-up).
Anyway, in first wave, just be careful of Bloat (and group of gorefast). Usually, if you headshot the bload with your knife, you will not get any damage. But, if you miss... So, sometimes, better to use your 9mm for it.
Other Tips> If you want to level up (demo for instance), you can start as a demo, kill everything with your knife... then in wave 2, change to Berserker (if you're at least level 10 with it !). Surprise : The knife is still the demo knife so you GET XP as a demo, not as a Berserker ! Better, you still get your bonus.
You can then kill everything with your knife. Just use your HX-25 for Crawler (because they are resistant to slashing damage) and Siren (if you took the perk which decreases the damage from her scream of 40%, you can really go in its face, wait for her to stop screaming and kill it with one headshot of your HX-25).
I usually use this until Wave 3, get my RPG, stay as a Berserker for one more Wave, and then when I got enough monney, I go demo.
Hope it helps !
(of course, it works with every class, just get their knife at the beginning
)