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 $29.78   3.739%   $30.8935   $30.89   $30.90   $30.90  
  Purchase #2 $5.55   12.646%   $6.2519   $6.25   $6.25   $6.30  
  Purchase #3 $18.42   7.605%   $19.8208   $19.82   $19.80   $19.80  
  Purchase #4 $71.67   8.367%   $77.6666   $77.67   $77.65   $77.70  
  Purchase #5 $150.33   13.313%   $170.3434   $170.34   $170.35   $170.30  
  Purchase #6 $239.66   6.760%   $255.861   $255.86   $255.85   $255.90  
  Purchase #7 $19.47   9.760%   $21.3703   $21.37   $21.35   $21.40  
  7 purchase totals = $534.88       $582.2075   $582.20   $582.15   $582.30