When Amazon automatically places ads, whether to choose the default bid or set the bid by targeting group depends on your advertising goals and strategies. Both methods have their own advantages and disadvantages, as follows:

- Default bid: This is a simpler method. You only need to set a default CPC bid, and Amazon will automatically adjust the bid in each targeting group based on your settings and ad performance. The advantage of this method is that it is simple and easy to use, and you don't need to set each targeting group separately. However, you may not be able to have fine-grained bidding control for some targeting groups.

- Set bids by targeting group: This method allows you to set different CPC bids for different targeting groups (such as close match, broad match, similar products, related products, etc.). This allows more precise control of the bid and ad delivery effect of each targeting group. By setting different bids for different targeting groups, you can better optimize ad delivery and improve the conversion rate and return on investment of your ads. However, this method requires you to set up and manage each targeting group separately, which is relatively complicated.

To sum up, if you want to have more precise control over the ad bid and delivery effect, you can choose to set the bid by targeting group; if you want to keep it simple and easy to use, you can choose the default bid. At the same time, you can also adjust the bidding method and bid at any time according to the specific situation and needs of the advertising campaign.