Your friends are right, redstone dust is evil and it should be avoided at all costs when making command block contraptions. Redstone dust does cause lag and it can be unpredictable. There's a nice blog post explaining why this is the case, but suffice it to say that pretty much every expert map maker limits their use of redstone dust to very specific edge cases, and it's never used on high frequency lines.
So, what should you be using instead? For a clock, use a 20Hz clock. Better yet, start using 1.9 and use repeating command blocks. For turning on a redstone device, use setblock to place a redstone block or torch at the activation point. This is especially useful for when your command blocks are far away from any redstone contraption (as they should be).
The only actual redstone that you should have in your command block contraption is redstone blocks (for activating the command blocks), comparators (for testing for a success on a command block, but even these aren't necessary; use the stats command instead), and on the extremely rare occasion, a repeater. Also, sometimes a button or switch, but that should be given. But that's for 1.8, and in 1.9, the entire command block game changes. 1.9 will allow you to remove even more redstone, to the point where you don't need any (not even a switch or button) to do something useful. Again, you'll need to be able to interface with redstone devices from time to time, but as I said above, that should be done with something like a setblock command.
In the end, you should do your best to keep your redstone stuff and your command block stuff separated. There was a time when this wasn't possible, but that was ages and ages ago, when the command block was first introduced. New commands made it possible to separate your redstone and command blocks, and with the new features in 1.9, this becomes even easier to do; you no longer need to think about how to build a 20Hz clock, execution order is trivialized to block placement, and conditional execution becomes built in. Redstone dust is great for making interesting contraptions in survival, but it has no place in your creative mode command blocks.
Minecraft v1.13 has changed the game again. Now, you don't even need command blocks in your world, you can use functions instead and tag them to run either every tick, or when the world loads. (Admittedly, some of this was available in earlier versions, but 1.13 was a major update to commands which made for a compelling reason to move as much as possible to functions.)
Command blocks will still be necessary in some very narrow circumstances (conditional execution in some cases is still difficult using just functions), but other than prototyping, I would recommend using functions instead of command blocks for most purposes. There's loads of reasons for doing this, but among the top reasons I see are:
- Code reuse and deduplication
- Maintainability
- Ease of reading
Something to keep in mind, though, is that a number of commands have been removed/replaced in 1.13, such as the stats command mentioned above (the new version as part of the execute command is actually a lot easier and more intuitive now). The power of commands has grown substantially since the command block was introduced in 1.4, but redstone has only seen incremental improvements. Most of the improvements in redstone were with the addition of blocks aimed at eliminating the need for BUD switches (those blocks being daylight detector and observer blocks). Other than that, there isn't much in the way of new redstone, whereas commands have been evolving almost with every release.
Best Answer
On PC:
Hold Shift (or if you changed key bindings, the sneak key) while placing to be able to place redstone on blocks that you can interact with (furnace, crafting table, dispenser etc.).
On Pocket Edition:
Press the sneak button, it should look like this (it should be a dark circle with a light circle in it):
and not like this (it shouldn't be a dark circle only):
On console editions:
Press the right analog stick (until it clicks, down on D Pad on PS Vita) to toggle the Sneak mode.