If the garment is going to be worn by a doll, then it may not require blocking, however if you intend to display the garment flat, then blocking may be required. The type of material used to knit the garment may also determine whether you need to block. Pure wool and acrylic yarns are very forgiving in miniature work and do not often require to be blocked. Cotton can often benefit from blocking, however do check the dye is colourfast before you decide to block.
The photo below shows a section of knitting which would benefit from blocking. This example is knitted with cotton yarn.
On the other hand this example, knitted with acrylic yarn would not require blocking.