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 $31.34   6.475%   $33.3693   $33.37   $33.35   $33.40  
  Purchase #2 $220.48   10.587%   $243.8222   $243.82   $243.80   $243.80  
  Purchase #3 $60.85   12.862%   $68.6765   $68.68   $68.70   $68.70  
  Purchase #4 $219.63   11.849%   $245.654   $245.65   $245.65   $245.70  
  Purchase #5 $26.94   12.116%   $30.2041   $30.20   $30.20   $30.20  
  Purchase #6 $116.29   12.550%   $130.8844   $130.88   $130.90   $130.90  
  Purchase #7 $243.30   10.511%   $268.8733   $268.87   $268.85   $268.90  
  7 purchase totals = $918.83       $1,021.4838   $1,021.47   $1,021.45   $1,021.60