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 $12.78   7.537%   $13.7432   $13.74   $13.75   $13.70  
  Purchase #2 $238.56   5.140%   $250.822   $250.82   $250.80   $250.80  
  Purchase #3 $137.46   7.822%   $148.2121   $148.21   $148.20   $148.20  
  Purchase #4 $138.46   13.391%   $157.0012   $157.00   $157.00   $157.00  
  Purchase #5 $127.36   12.281%   $143.0011   $143.00   $143.00   $143.00  
  Purchase #6 $14.94   13.865%   $17.0114   $17.01   $17.00   $17.00  
  Purchase #7 $123.91   3.527%   $128.2803   $128.28   $128.30   $128.30  
  7 purchase totals = $793.47       $858.0713   $858.06   $858.05   $858.00