I'm currently level 29, and level 38 in smithing. When should I start spending perks into it? I don't want to do what I've done on a previous character and focused on smithing only to find I'm terribly underpowered in actual combat...

Oh, and should I invest in enchanting, too? My level is 58.

Anyone?

Whenever you plan on using it. Smithing isn't going to make you weaker unless you fail to use what you make.

While that's true, I'll have less perks invested into combat, thus making me less efficient in combat.

"Combat" perks raise your armor and weapon damage.
Smithing raises... your armor and weapon damage.

I am not entire sure how you can make that claim.

Your missing the balance here. Yes, you might be weaker in combat, but you do more damage per hit with a better weapon.

Another way to look at it, you don't need any Perks to be successful in this game, skills will get you where you want to be. So, to that end, it really does not matter where you use your Perk points.

You'd be surprised to find out that 100 alch/ench/smith can result in a stronger "base combatant" than fully speccing in 3 combat skills.

For example, I took a base character, advskilled all the crafting to 100, and added the needed mats. I created a "perfect" set of daedric armour/shield/longsword/ring/amulet, and enchanted it with fortify heavy armour and fortify one handed (all of this done under the effects of maxed out alchemy potions).

On its own, the crafted gear resulted in about 690 armour, 140 attack.

Then I took a base character, advskilled Heavy Armour, Block, and one handed, and equipped with "base" daedric gear. 490ish armour, 40 attack.

Crafting > skill. Or rather, it is a matter of having the coolest toys, not a matter of knowing how to use em.

Hmmm... should I powerlevel with that iron dagger thing?

When you are planning on improving or making armor that uses the perk.

Did you raid a dwemer ruin and have materials for dwarven armor? Assign the perk.
Find an orcish weapon that needs sharpened? Assign the perk.
Hit smithing 90 but don't have any daedra hearts? Don't assign the perk.

Interesting findings, but combat skills also add more than just to base damage and defense, they also give you different abilities, lower magic/stamina cost etc...

You can probably complete the game without assigning any of your Perks.

In fact, it might make it more fun and balanced.

There is no chance it will make you underpowered.

I guess since you are level 29 already, the first 3 perks might not be useful since your gear is already higher level. Whatever. 3 levels of wasted perks is going to be comnpletely unnoticable to your power level.

At level 29 alot of the scaled enemy gear is going to be as good as alot of stuff you can make. Unless you are ok with burning points in the smithing tree in order to unlock the top teir armor like Dragon / Ebony / Daedric. Being able to upgrade higher teir armor does make a large difference though!

Well don't do that either, as it then becomes more difficult to acquire gold/loot so as to buy the actual materials you need to craft. Use a more balanced approach. Generally, try to make sure you smithing level is roughly equal to the type of gear you're finding at your current level (or only slightly better). That is to say, if you're often finding dwemer armour, make sure you have the dwemer perk. If you're finding orcish armour, make sure you have the orcish perk, etc. It doesn't hurt to "skip ahead" a bit, but don't go too far. Try to make sure you can at least buy the materials for the new type of armour you will be able to craft.

This is how I did it:

Only put points in Smithing if you actually have the gear to upgrade. For instance, if you haven't found a single piece of Orcish gear, then don't bother perking it in the Smithing tree until later.

I don't actually make my own gear though, since I'm only using gear I find in dungeons or I buy from merchants. Having a piece of Dwarven gear pre-enchanted seems like a better option than a non-enchanted (or weak-enchanted) piece of ebony gear. My enchanting is 80 and I still can't make any enchanted gear that comes close to the random gear I've found. About 8% vs. 25% boosts respectively.