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 $128.71   9.496%   $140.9323   $140.93   $140.95   $140.90  
  Purchase #2 $71.25   12.792%   $80.3643   $80.36   $80.35   $80.40  
  Purchase #3 $128.28   3.559%   $132.8455   $132.85   $132.85   $132.80  
  Purchase #4 $22.78   11.264%   $25.3459   $25.35   $25.35   $25.30  
  Purchase #5 $146.84   11.246%   $163.3536   $163.35   $163.35   $163.40  
  Purchase #6 $27.46   5.761%   $29.042   $29.04   $29.05   $29.00  
  Purchase #7 $105.36   12.308%   $118.3277   $118.33   $118.35   $118.30  
  7 purchase totals = $630.68       $690.2113   $690.21   $690.25   $690.10