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 $7.91   12.349%   $8.8868   $8.89   $8.90   $8.90  
  Purchase #2 $162.72   3.755%   $168.8301   $168.83   $168.85   $168.80  
  Purchase #3 $133.97   9.246%   $146.3569   $146.36   $146.35   $146.40  
  Purchase #4 $155.92   6.170%   $165.5403   $165.54   $165.55   $165.50  
  Purchase #5 $80.58   9.430%   $88.1787   $88.18   $88.20   $88.20  
  Purchase #6 $112.97   3.304%   $116.7025   $116.70   $116.70   $116.70  
  Purchase #7 $164.26   3.587%   $170.152   $170.15   $170.15   $170.20  
  7 purchase totals = $818.33       $864.6473   $864.65   $864.70   $864.70