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 $20.50   3.294%   $21.1753   $21.18   $21.20   $21.20  
  Purchase #2 $13.97   6.206%   $14.837   $14.84   $14.85   $14.80  
  Purchase #3 $233.42   8.313%   $252.8242   $252.82   $252.80   $252.80  
  Purchase #4 $102.41   7.396%   $109.9842   $109.98   $110.00   $110.00  
  Purchase #5 $143.93   10.374%   $158.8613   $158.86   $158.85   $158.90  
  Purchase #6 $5.70   7.568%   $6.1314   $6.13   $6.15   $6.10  
  Purchase #7 $38.83   8.748%   $42.2268   $42.23   $42.25   $42.20  
  7 purchase totals = $558.76       $606.0402   $606.04   $606.10   $606.00