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 $243.95   13.156%   $276.0441   $276.04   $276.05   $276.00  
  Purchase #2 $223.57   10.766%   $247.6395   $247.64   $247.65   $247.60  
  Purchase #3 $97.48   13.533%   $110.672   $110.67   $110.65   $110.70  
  Purchase #4 $38.76   12.729%   $43.6938   $43.69   $43.70   $43.70  
  Purchase #5 $82.66   9.487%   $90.502   $90.50   $90.50   $90.50  
  Purchase #6 $181.73   10.413%   $200.6535   $200.65   $200.65   $200.70  
  Purchase #7 $86.25   9.576%   $94.5093   $94.51   $94.50   $94.50  
  7 purchase totals = $954.40       $1,063.7142   $1,063.70   $1,063.70   $1,063.70