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 $53.81   11.181%   $59.8265   $59.83   $59.85   $59.80  
  Purchase #2 $217.55   12.136%   $243.9519   $243.95   $243.95   $244.00  
  Purchase #3 $25.40   13.465%   $28.8201   $28.82   $28.80   $28.80  
  Purchase #4 $16.35   6.570%   $17.4242   $17.42   $17.40   $17.40  
  Purchase #5 $50.48   5.805%   $53.4104   $53.41   $53.40   $53.40  
  Purchase #6 $65.99   12.417%   $74.184   $74.18   $74.20   $74.20  
  Purchase #7 $71.57   3.144%   $73.8202   $73.82   $73.80   $73.80  
  7 purchase totals = $501.15       $551.4373   $551.43   $551.40   $551.40