Background/Objectives: Bite blocks (BBs) are commonly used in clear aligner therapy, yet their effect on masticatory muscle activity is poorly characterised. This study aims to examine the variability in the effects of anterior and posterior BBs on masticatory muscle activity in healthy participants. Methods: Twenty-four healthy adults were included in a crossover design comparing anterior and posterior BBs in removable passive aligners. Surface electromyography measured muscle activity at baseline, 24 h, 48 h and one week. Participants completed psychosocial questionnaires. Response patterns were identified via cluster analysis, and psychosocial differences were assessed with t-tests. Results: Anterior BBs immediately reduced muscle work intensity, with significant inter-individual variability (p = 0.030 at T1; p = 0.047 at T3). In contrast, posterior BBs altered muscle activity without significant variability over time (p > 0.05). Cluster analysis applied to anterior BB revealed two distinct adaptation patterns, which had significantly different Somatosensory Amplification Scale scores (p = 0.040). Conclusions: Within the limitations of the study, in the same participants, wearing passive aligners with anterior bite blocks significantly increased the inter-individual variability of muscular work intensity relative to the no-appliance baseline, whereas passive aligners with posterior bite blocks produced no significant change in variability.