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 $37.41   4.385%   $39.0504   $39.05   $39.05   $39.10  
  Purchase #2 $95.76   11.220%   $106.5043   $106.50   $106.50   $106.50  
  Purchase #3 $121.65   9.260%   $132.9148   $132.91   $132.90   $132.90  
  Purchase #4 $78.53   6.799%   $83.8693   $83.87   $83.85   $83.90  
  Purchase #5 $138.93   10.528%   $153.5566   $153.56   $153.55   $153.60  
  Purchase #6 $14.55   4.706%   $15.2347   $15.23   $15.25   $15.20  
  Purchase #7 $99.66   9.456%   $109.0838   $109.08   $109.10   $109.10  
  7 purchase totals = $586.49       $640.2139   $640.20   $640.20   $640.30