There're some special priciples for the use of pinyin consonant "ü"

When "ü" follows “y, j, q, x”, it becomes “u”.

For instance,
y:yún(云)
    yúan(圆)
j:jù(句)
    júe(觉)
q:qùe(雀)
    qù(去)
x:xuǎn(选)
     xū(需)

When "ü" follows “n, l”, the dots can not be omitted.

For instance,
n: nǚ(女)
l:lǜ(绿)
   lǚ(吕).

That's the rules when to omit the  dots above "ü".