You should shim whichever side of the toilet has a gap causing the rock, which is most commonly the front or sides of the porcelain bowl due to uneven tile or flooring slopes. Plumbers generally advise placing shims symmetrically under the lower contact points around the base to ensure the bowl is fully supported. Never leave a gap unsupported, and avoid over-tightening the floor bolts to pull the toilet down, as this cracks the ceramic. Verification: Check that all shims make firm contact and the toilet does not rock in any direction.
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