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 $128.75   13.576%   $146.2291   $146.23   $146.25   $146.20  
  Purchase #2 $174.88   10.156%   $192.6408   $192.64   $192.65   $192.60  
  Purchase #3 $63.41   7.377%   $68.0878   $68.09   $68.10   $68.10  
  Purchase #4 $11.92   12.113%   $13.3639   $13.36   $13.35   $13.40  
  Purchase #5 $98.61   3.575%   $102.1353   $102.14   $102.15   $102.10  
  Purchase #6 $233.80   11.170%   $259.9155   $259.92   $259.90   $259.90  
  Purchase #7 $25.52   7.842%   $27.5213   $27.52   $27.50   $27.50  
  7 purchase totals = $736.89       $809.8937   $809.90   $809.90   $809.80