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 $190.88   13.495%   $216.6393   $216.64   $216.65   $216.60  
  Purchase #2 $77.51   7.189%   $83.0822   $83.08   $83.10   $83.10  
  Purchase #3 $39.85   11.843%   $44.5694   $44.57   $44.55   $44.60  
  Purchase #4 $64.41   13.443%   $73.0686   $73.07   $73.05   $73.10  
  Purchase #5 $43.50   7.774%   $46.8817   $46.88   $46.90   $46.90  
  Purchase #6 $83.31   12.585%   $93.7946   $93.79   $93.80   $93.80  
  Purchase #7 $104.95   3.626%   $108.7555   $108.76   $108.75   $108.80  
  7 purchase totals = $604.41       $666.7913   $666.79   $666.80   $666.90