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".