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 $70.68   13.128%   $79.9589   $79.96   $79.95   $80.00  
  Purchase #2 $54.53   11.309%   $60.6968   $60.70   $60.70   $60.70  
  Purchase #3 $229.32   11.507%   $255.7079   $255.71   $255.70   $255.70  
  Purchase #4 $150.86   3.654%   $156.3724   $156.37   $156.35   $156.40  
  Purchase #5 $229.63   3.285%   $237.1733   $237.17   $237.15   $237.20  
  Purchase #6 $70.72   4.711%   $74.0516   $74.05   $74.05   $74.10  
  Purchase #7 $209.91   13.808%   $238.8944   $238.89   $238.90   $238.90  
  7 purchase totals = $1,015.65       $1,102.8553   $1,102.85   $1,102.80   $1,103.00