What your mechanic was referring to was getting any silicone into the intake can screw up (poison) your O2 sensor. This includes any silicone from excessive RTV on the intake and silicate from coolant. Otherwise, spray away.
Tip: I keep a tube of silicone (synonymous with "dielectric grease") in my toolbox. It's a must to coat the inside of ignition wire boots where they contact the spark plug ceramic.....a must for the sliding elements of brake calipers, and useful for all electrical connections to exclude corrosive environment.
95 E320 Cabriolet, 131K