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 $75.48   4.589%   $78.9438   $78.94   $78.95   $78.90  
  Purchase #2 $26.72   12.132%   $29.9617   $29.96   $29.95   $30.00  
  Purchase #3 $16.43   9.391%   $17.9729   $17.97   $17.95   $18.00  
  Purchase #4 $17.46   3.445%   $18.0615   $18.06   $18.05   $18.10  
  Purchase #5 $172.40   7.393%   $185.1455   $185.15   $185.15   $185.10  
  Purchase #6 $64.44   7.242%   $69.1067   $69.11   $69.10   $69.10  
  Purchase #7 $48.77   10.845%   $54.0591   $54.06   $54.05   $54.10  
  7 purchase totals = $421.70       $453.2512   $453.25   $453.20   $453.30