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 $24.31   12.569%   $27.3655   $27.37   $27.35   $27.40  
  Purchase #2 $118.72   9.731%   $130.2726   $130.27   $130.25   $130.30  
  Purchase #3 $224.99   8.758%   $244.6946   $244.69   $244.70   $244.70  
  Purchase #4 $249.43   8.836%   $271.4696   $271.47   $271.45   $271.50  
  Purchase #5 $139.59   9.518%   $152.8762   $152.88   $152.90   $152.90  
  Purchase #6 $235.27   10.495%   $259.9616   $259.96   $259.95   $260.00  
  Purchase #7 $243.80   4.164%   $253.9518   $253.95   $253.95   $254.00  
  7 purchase totals = $1,236.11       $1,340.5919   $1,340.59   $1,340.55   $1,340.80