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 $147.83   12.669%   $166.5586   $166.56   $166.55   $166.60  
  Purchase #2 $120.29   8.314%   $130.2909   $130.29   $130.30   $130.30  
  Purchase #3 $53.82   6.344%   $57.2343   $57.23   $57.25   $57.20  
  Purchase #4 $106.39   12.498%   $119.6866   $119.69   $119.70   $119.70  
  Purchase #5 $125.64   10.236%   $138.5005   $138.50   $138.50   $138.50  
  Purchase #6 $48.69   4.418%   $50.8411   $50.84   $50.85   $50.80  
  Purchase #7 $137.32   13.446%   $155.784   $155.78   $155.80   $155.80  
  7 purchase totals = $739.98       $818.8960   $818.89   $818.95   $818.90