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 $26.47   5.770%   $27.9973   $28.00   $28.00   $28.00  
  Purchase #2 $103.37   4.585%   $108.1095   $108.11   $108.10   $108.10  
  Purchase #3 $70.87   7.400%   $76.1144   $76.11   $76.10   $76.10  
  Purchase #4 $58.77   8.808%   $63.9465   $63.95   $63.95   $63.90  
  Purchase #5 $155.96   9.752%   $171.1692   $171.17   $171.15   $171.20  
  Purchase #6 $74.85   10.144%   $82.4428   $82.44   $82.45   $82.40  
  Purchase #7 $25.80   12.666%   $29.0678   $29.07   $29.05   $29.10  
  7 purchase totals = $516.09       $558.8475   $558.85   $558.80   $558.80