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 $206.43   12.639%   $232.5207   $232.52   $232.50   $232.50  
  Purchase #2 $250.84   13.262%   $284.1064   $284.11   $284.10   $284.10  
  Purchase #3 $19.56   4.408%   $20.4222   $20.42   $20.40   $20.40  
  Purchase #4 $97.83   13.118%   $110.6633   $110.66   $110.65   $110.70  
  Purchase #5 $163.83   7.715%   $176.4695   $176.47   $176.45   $176.50  
  Purchase #6 $56.79   10.703%   $62.8682   $62.87   $62.85   $62.90  
  Purchase #7 $165.56   4.337%   $172.7403   $172.74   $172.75   $172.70  
  7 purchase totals = $960.84       $1,059.7906   $1,059.79   $1,059.70   $1,059.80