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 $7.80   8.175%   $8.4377   $8.44   $8.45   $8.40  
  Purchase #2 $23.69   6.499%   $25.2296   $25.23   $25.25   $25.20  
  Purchase #3 $209.96   6.127%   $222.8242   $222.82   $222.80   $222.80  
  Purchase #4 $57.97   7.650%   $62.4047   $62.40   $62.40   $62.40  
  Purchase #5 $127.55   5.225%   $134.2145   $134.21   $134.20   $134.20  
  Purchase #6 $205.69   8.624%   $223.4287   $223.43   $223.45   $223.40  
  Purchase #7 $11.28   7.831%   $12.1633   $12.16   $12.15   $12.20  
  7 purchase totals = $643.94       $688.7027   $688.69   $688.70   $688.60