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 $59.69   10.770%   $66.1186   $66.12   $66.10   $66.10  
  Purchase #2 $173.53   11.585%   $193.6335   $193.63   $193.65   $193.60  
  Purchase #3 $250.94   7.451%   $269.6375   $269.64   $269.65   $269.60  
  Purchase #4 $156.83   9.403%   $171.5767   $171.58   $171.60   $171.60  
  Purchase #5 $120.78   3.573%   $125.0955   $125.10   $125.10   $125.10  
  Purchase #6 $30.95   13.740%   $35.2025   $35.20   $35.20   $35.20  
  Purchase #7 $104.28   13.807%   $118.6779   $118.68   $118.70   $118.70  
  7 purchase totals = $897.00       $979.9422   $979.95   $980.00   $979.90