Here's a little experiment for you. Brew a pot of tea. Let it cool. Now remove half the pot and put it in a container in the fridge. Leave the other half out on the counter. Now let's see how quickly it gets too gross to drink. If it's anything like my house in the summer, it could be as short as two days before we see little clumps of things coagulating in the counter version. It might be up to a week for the version in the fridge, but you'll soon see little bits of stuff floating in it. That's ick. (For a fun variation, stick your finger in them now and again and see if this changes the rate of ick!)
This is one of the reasons I'm not a fan of making our own infusions or using tea in our products - even experienced formulators have trouble preserving these kinds of liquids!
Would you drink either of these if they were treated improperly? Although it's hard to compare pots of tea with body wash, for instance, we are using botanical ingredients like those found in some food stuffs - extracts, aloe vera, oils, and so on - that can go off very quickly if not treated properly.
Preservatives are not optional.
This brings me to the question of re-using containers. I know when you are experimenting it can get a bit expensive to buy containers you are only using once, but there's a good reason for this. Our containers are plastic, and they absorb the oils and other things we use in our creations.
When I first started, I re-used containers constantly. I bought some really nice 8 ounce containers for sugar scrubs, and ran them through the dishwasher after the first batch. I thought they were clean - until I smelled rancidity only a few weeks after my second batch. Yep, the first batch was still in the plastic, lurking about, causing my lovely scrub to smell just horrible.
I admit I re-fill my body wash and leave-in conditioner containers a few times before recycling them...I know this isn't the best practice, and I hang my head in shame. My logic is that if I am using very small containers with non-oil containing products that are used up quickly, I'm probably okay. But let's me honest - it's not the right thing to do. Imagine if I mixed up one of those many-times-used containers with a new one and gave it to someone?
If you are worried about spending too much on containers, invest in some canning jars or other glass containers for your oil containing creations. Yes, glass isn't the best choice in a slippery environment like the bathroom, but it's better than the stench of rancidity or the fun of a microbial infection!