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 $51.83   9.159%   $56.5771   $56.58   $56.60   $56.60  
  Purchase #2 $172.64   7.764%   $186.0438   $186.04   $186.05   $186.00  
  Purchase #3 $170.43   5.713%   $180.1667   $180.17   $180.15   $180.20  
  Purchase #4 $189.46   6.573%   $201.9132   $201.91   $201.90   $201.90  
  Purchase #5 $76.98   12.445%   $86.5602   $86.56   $86.55   $86.60  
  Purchase #6 $54.75   7.120%   $58.6482   $58.65   $58.65   $58.60  
  Purchase #7 $8.54   6.712%   $9.1132   $9.11   $9.10   $9.10  
  7 purchase totals = $724.63       $779.0224   $779.02   $779.00   $779.00