Thats the general concensus with horde armies, or just have a million guys plowing fire into enemy troops and tanks, although obviously Orks arent too good at that.... And 'Nids dont have much shooty tank-busting ability other than Zoanthropes
Lots of termagants with fleshborers can make a right mess!
My friend Jon plays Orks. I've observed him and here's a few things I've learned.
1. If you have to make a choice, move + run instead of shooting with your troops. Let your elites take care of shooting, da boyz should always be moving forward towards the enemy.
2. Distrust the Shokk Attack Gun. I've seen some majorly bad rolls lose games for Jon more times than good rolls have won them.
3. Buy a graphics card for your best friend so he'll repay you by buying you $600 worth of Orks (IRL example, you're welcome Jon.)
4. Measure and cut out a 2" stick of plastic from any leftover sprues you have. As Orks, you need to space out as much as possible against pie-loving enemies like the Guard and, to a lesser extent, Tau. Use the stick to make sure those pie plates are bagging as few of your boys as possible.
5. Stack your attack like this- Full Boyz mobs up front (no special weapons), Elites behind them, 'ard Stuff in the rear. Your boys will be chewed up a bit on the slog over, but they'll smash into the other line and take fire off your elites and 'ard stuff. As soon as your Boyz drop, attack with your Elites, Kans, Dreds, Warboss, and anything else you cant throw at them.
As thenoblewolves mentioned, squad cover is always good. With nids, I have rippers giving cover to gaunts/genestealers, gaunts/genestealers to warriors/lictors/raveners/tryrant guard and warriors/lictors/raveners/tryrant guard giving cover to MCs. With the Tyrant guard its especially effective, as with the gaunts/lictor combo ;D