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 $25.94   13.406%   $29.4175   $29.42   $29.40   $29.40  
  Purchase #2 $179.74   5.332%   $189.3237   $189.32   $189.30   $189.30  
  Purchase #3 $47.46   8.370%   $51.4324   $51.43   $51.45   $51.40  
  Purchase #4 $74.72   7.412%   $80.2582   $80.26   $80.25   $80.30  
  Purchase #5 $174.45   4.516%   $182.3282   $182.33   $182.35   $182.30  
  Purchase #6 $63.87   3.685%   $66.2236   $66.22   $66.20   $66.20  
  Purchase #7 $106.25   12.626%   $119.6651   $119.67   $119.65   $119.70  
  7 purchase totals = $672.43       $718.6487   $718.65   $718.60   $718.60