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 $216.73   12.241%   $243.2599   $243.26   $243.25   $243.30  
  Purchase #2 $8.49   3.347%   $8.7742   $8.77   $8.75   $8.80  
  Purchase #3 $203.28   4.257%   $211.9336   $211.93   $211.95   $211.90  
  Purchase #4 $35.69   4.580%   $37.3246   $37.32   $37.30   $37.30  
  Purchase #5 $237.59   10.479%   $262.4871   $262.49   $262.50   $262.50  
  Purchase #6 $60.83   7.732%   $65.5334   $65.53   $65.55   $65.50  
  Purchase #7 $8.38   10.817%   $9.2865   $9.29   $9.30   $9.30  
  7 purchase totals = $770.99       $838.5993   $838.59   $838.60   $838.60