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 $58.91   13.360%   $66.7804   $66.78   $66.80   $66.80  
  Purchase #2 $200.57   13.416%   $227.4785   $227.48   $227.50   $227.50  
  Purchase #3 $143.35   8.732%   $155.8673   $155.87   $155.85   $155.90  
  Purchase #4 $216.27   13.623%   $245.7325   $245.73   $245.75   $245.70  
  Purchase #5 $27.47   11.536%   $30.6389   $30.64   $30.65   $30.60  
  Purchase #6 $123.85   4.352%   $129.24   $129.24   $129.25   $129.20  
  Purchase #7 $144.75   13.751%   $164.6546   $164.65   $164.65   $164.70  
  7 purchase totals = $915.17       $1,020.3922   $1,020.39   $1,020.45   $1,020.40