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 $7.75   8.586%   $8.4154   $8.42   $8.40   $8.40  
  Purchase #2 $8.83   6.230%   $9.3801   $9.38   $9.40   $9.40  
  Purchase #3 $138.70   7.300%   $148.8251   $148.83   $148.85   $148.80  
  Purchase #4 $48.79   11.752%   $54.5238   $54.52   $54.50   $54.50  
  Purchase #5 $148.54   5.819%   $157.1835   $157.18   $157.20   $157.20  
  Purchase #6 $196.74   13.513%   $223.3255   $223.33   $223.35   $223.30  
  Purchase #7 $73.73   9.212%   $80.522   $80.52   $80.50   $80.50  
  7 purchase totals = $623.08       $682.1754   $682.18   $682.20   $682.10