The best solution would be to use margin: auto; on the flex items horizontal margins as follows:
.flexbox {
background-color: pink;
display: flex;
justify-content: space-between;
flex-wrap: wrap;
}
.flex-item {
width: 175px;
height: 175px;
background-color: grey;
margin: 10px auto;
}
<div class="flexbox">
<div class="flex-item"></div>
<div class="flex-item"></div>
<div class="flex-item"></div>
<div class="flex-item"></div>
<div class="flex-item"></div>
<div class="flex-item"></div>
<div class="flex-item"></div>
</div>
The only problem might be if the last row would have more than one item but not enough to fill up the whole row, there will be a lot of space between the last row items.