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 $232.42   11.539%   $259.2389   $259.24   $259.25   $259.20  
  Purchase #2 $183.95   7.461%   $197.6745   $197.67   $197.65   $197.70  
  Purchase #3 $43.50   7.438%   $46.7355   $46.74   $46.75   $46.70  
  Purchase #4 $53.89   9.852%   $59.1992   $59.20   $59.20   $59.20  
  Purchase #5 $37.55   5.278%   $39.5319   $39.53   $39.55   $39.50  
  Purchase #6 $51.30   4.564%   $53.6413   $53.64   $53.65   $53.60  
  Purchase #7 $247.79   10.797%   $274.5439   $274.54   $274.55   $274.50  
  7 purchase totals = $850.40       $930.5652   $930.56   $930.60   $930.40