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 $100.83   5.666%   $106.543   $106.54   $106.55   $106.50  
  Purchase #2 $39.28   12.552%   $44.2104   $44.21   $44.20   $44.20  
  Purchase #3 $7.96   13.450%   $9.0306   $9.03   $9.05   $9.00  
  Purchase #4 $152.30   12.466%   $171.2857   $171.29   $171.30   $171.30  
  Purchase #5 $27.62   8.496%   $29.9666   $29.97   $29.95   $30.00  
  Purchase #6 $125.48   6.232%   $133.2999   $133.30   $133.30   $133.30  
  Purchase #7 $149.41   3.137%   $154.097   $154.10   $154.10   $154.10  
  7 purchase totals = $602.88       $648.4332   $648.44   $648.45   $648.40