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 $58.97   12.855%   $66.5506   $66.55   $66.55   $66.60  
  Purchase #2 $42.74   13.223%   $48.3915   $48.39   $48.40   $48.40  
  Purchase #3 $152.41   12.536%   $171.5161   $171.52   $171.50   $171.50  
  Purchase #4 $210.87   5.865%   $223.2375   $223.24   $223.25   $223.20  
  Purchase #5 $181.35   11.718%   $202.6006   $202.60   $202.60   $202.60  
  Purchase #6 $7.76   12.813%   $8.7543   $8.75   $8.75   $8.80  
  Purchase #7 $30.81   3.777%   $31.9737   $31.97   $31.95   $32.00  
  7 purchase totals = $684.91       $753.0243   $753.02   $753.00   $753.10