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 $25.55   11.195%   $28.4103   $28.41   $28.40   $28.40  
  Purchase #2 $133.61   4.139%   $139.1401   $139.14   $139.15   $139.10  
  Purchase #3 $187.44   9.547%   $205.3349   $205.33   $205.35   $205.30  
  Purchase #4 $55.31   6.385%   $58.8415   $58.84   $58.85   $58.80  
  Purchase #5 $219.33   5.858%   $232.1784   $232.18   $232.20   $232.20  
  Purchase #6 $234.79   4.373%   $245.0574   $245.06   $245.05   $245.10  
  Purchase #7 $28.36   7.199%   $30.4016   $30.40   $30.40   $30.40  
  7 purchase totals = $884.39       $939.3642   $939.36   $939.40   $939.30