Let's do some math ..  Rounding examples of 7 random hypothetical purchases:
  Case Tag  
Price  
Sales  
Tax %  
Raw Total  
Incl. Sales Tax  
Rounded  
to Penny  
Rounded  
to Nickel  
Rounded  
to Dime  
  Purchase #1 $7.74   4.348%   $8.0765   $8.08   $8.10   $8.10  
  Purchase #2 $137.66   8.518%   $149.3859   $149.39   $149.40   $149.40  
  Purchase #3 $145.52   4.175%   $151.5955   $151.60   $151.60   $151.60  
  Purchase #4 $20.25   13.330%   $22.9493   $22.95   $22.95   $22.90  
  Purchase #5 $162.71   4.286%   $169.6838   $169.68   $169.70   $169.70  
  Purchase #6 $51.38   7.439%   $55.2022   $55.20   $55.20   $55.20  
  Purchase #7 $73.30   13.780%   $83.4007   $83.40   $83.40   $83.40  
  7 purchase totals = $598.56       $640.2939   $640.30   $640.35   $640.30