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 $227.96   13.679%   $259.1426   $259.14   $259.15   $259.10  
  Purchase #2 $181.33   8.713%   $197.1293   $197.13   $197.15   $197.10  
  Purchase #3 $53.63   10.118%   $59.0563   $59.06   $59.05   $59.10  
  Purchase #4 $65.91   11.581%   $73.543   $73.54   $73.55   $73.50  
  Purchase #5 $19.83   6.203%   $21.0601   $21.06   $21.05   $21.10  
  Purchase #6 $158.69   5.273%   $167.0577   $167.06   $167.05   $167.10  
  Purchase #7 $203.28   11.520%   $226.6979   $226.70   $226.70   $226.70  
  7 purchase totals = $910.63       $1,003.6869   $1,003.69   $1,003.70   $1,003.70