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 $51.87   10.742%   $57.4419   $57.44   $57.45   $57.40  
  Purchase #2 $143.56   8.132%   $155.2343   $155.23   $155.25   $155.20  
  Purchase #3 $232.40   10.279%   $256.2884   $256.29   $256.30   $256.30  
  Purchase #4 $175.62   6.704%   $187.3936   $187.39   $187.40   $187.40  
  Purchase #5 $72.90   8.600%   $79.1694   $79.17   $79.15   $79.20  
  Purchase #6 $46.37   6.339%   $49.3094   $49.31   $49.30   $49.30  
  Purchase #7 $209.29   8.232%   $226.5188   $226.52   $226.50   $226.50  
  7 purchase totals = $932.01       $1,011.3558   $1,011.35   $1,011.35   $1,011.30