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 $26.60   7.592%   $28.6195   $28.62   $28.60   $28.60  
  Purchase #2 $8.73   4.169%   $9.094   $9.09   $9.10   $9.10  
  Purchase #3 $222.78   12.503%   $250.6342   $250.63   $250.65   $250.60  
  Purchase #4 $31.63   9.262%   $34.5596   $34.56   $34.55   $34.60  
  Purchase #5 $193.31   8.390%   $209.5287   $209.53   $209.55   $209.50  
  Purchase #6 $40.84   12.261%   $45.8474   $45.85   $45.85   $45.80  
  Purchase #7 $230.81   5.336%   $243.126   $243.13   $243.15   $243.10  
  7 purchase totals = $754.70       $821.4094   $821.41   $821.45   $821.30