It's generally best to buy and use single-ingredient supplements. There are several reasons for this.
First, you are more likely to get what's listed on the label with a single-ingredient supplement. Years of testing by ConsumerLab.com have shown that combination products are more likely to contain incorrect amounts of one or more ingredients -- and they are more likely to be contaminated.
Second, with a single-ingredient supplement, the recommended daily serving of that ingredient is more likely to be clinically relevant than if the ingredient is part of a combination formula, where a tiny amount may be included merely as "window dressing." In addition, most combinations on the market have not been clinically tested.
Third, using single-ingredient supplements gives you a better idea of which ingredients work and which do not, as well as which are causing side-effects. This allows you to adjust your supplements and dosage to better suit your needs.
Finally, you may find it actually costs less
to purchase ingredients as separate supplements than in a combination formula. Many single-ingredient products of high quality and low cost are identified in the ConsumerLab.com's Product Reviews
With that said, an appropriate time to use a combination formula is when that exact combination has been shown to work in a well-controlled clinical trial and it is less expensive to purchase (or much more convenient to use) than the corresponding collection of single-ingredient supplements.