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 $103.50   13.348%   $117.3152   $117.32   $117.30   $117.30  
  Purchase #2 $204.38   9.829%   $224.4685   $224.47   $224.45   $224.50  
  Purchase #3 $115.62   4.112%   $120.3743   $120.37   $120.35   $120.40  
  Purchase #4 $53.53   9.159%   $58.4328   $58.43   $58.45   $58.40  
  Purchase #5 $117.52   9.800%   $129.037   $129.04   $129.05   $129.00  
  Purchase #6 $113.61   5.406%   $119.7518   $119.75   $119.75   $119.80  
  Purchase #7 $131.94   5.128%   $138.7059   $138.71   $138.70   $138.70  
  7 purchase totals = $840.10       $908.0855   $908.09   $908.05   $908.10