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 $18.99   8.816%   $20.6642   $20.66   $20.65   $20.70  
  Purchase #2 $140.89   13.563%   $159.9989   $160.00   $160.00   $160.00  
  Purchase #3 $158.82   8.418%   $172.1895   $172.19   $172.20   $172.20  
  Purchase #4 $76.36   4.365%   $79.6931   $79.69   $79.70   $79.70  
  Purchase #5 $154.60   12.682%   $174.2064   $174.21   $174.20   $174.20  
  Purchase #6 $241.35   5.168%   $253.823   $253.82   $253.80   $253.80  
  Purchase #7 $108.31   11.410%   $120.6682   $120.67   $120.65   $120.70  
  7 purchase totals = $899.32       $981.2433   $981.24   $981.20   $981.30