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 $179.28   13.592%   $203.6477   $203.65   $203.65   $203.60  
  Purchase #2 $158.39   4.647%   $165.7504   $165.75   $165.75   $165.80  
  Purchase #3 $182.98   4.221%   $190.7036   $190.70   $190.70   $190.70  
  Purchase #4 $116.99   4.217%   $121.9235   $121.92   $121.90   $121.90  
  Purchase #5 $42.49   6.250%   $45.1456   $45.15   $45.15   $45.10  
  Purchase #6 $132.70   11.534%   $148.0056   $148.01   $148.00   $148.00  
  Purchase #7 $150.26   9.681%   $164.8067   $164.81   $164.80   $164.80  
  7 purchase totals = $963.09       $1,039.9831   $1,039.99   $1,039.95   $1,039.90