Are there any rules for omitting the * operator in CAS

[parisse]

ax can never be used for a*x inside the CAS, because otherwise variable names would be restricted to one letter.
You can omit * in situations like number*variable: 2x is ok.
If you don't know, write a * : it won't hurt if you enter 2*x, and this is a good habit if you are going to program.