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 $9.59   9.220%   $10.4742   $10.47   $10.45   $10.50  
  Purchase #2 $61.72   7.146%   $66.1305   $66.13   $66.15   $66.10  
  Purchase #3 $17.28   11.539%   $19.2739   $19.27   $19.25   $19.30  
  Purchase #4 $115.50   9.743%   $126.7532   $126.75   $126.75   $126.80  
  Purchase #5 $130.39   12.408%   $146.5688   $146.57   $146.55   $146.60  
  Purchase #6 $89.98   12.771%   $101.4713   $101.47   $101.45   $101.50  
  Purchase #7 $68.91   9.470%   $75.4358   $75.44   $75.45   $75.40  
  7 purchase totals = $493.37       $546.1077   $546.10   $546.05   $546.20