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 $55.39   6.598%   $59.0446   $59.04   $59.05   $59.00  
  Purchase #2 $244.85   7.409%   $262.9909   $262.99   $263.00   $263.00  
  Purchase #3 $163.63   4.217%   $170.5303   $170.53   $170.55   $170.50  
  Purchase #4 $99.28   4.114%   $103.3644   $103.36   $103.35   $103.40  
  Purchase #5 $93.44   4.502%   $97.6467   $97.65   $97.65   $97.60  
  Purchase #6 $75.70   12.218%   $84.949   $84.95   $84.95   $84.90  
  Purchase #7 $198.87   6.244%   $211.2874   $211.29   $211.30   $211.30  
  7 purchase totals = $931.16       $989.8133   $989.81   $989.85   $989.70