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 $24.43   12.442%   $27.4696   $27.47   $27.45   $27.50  
  Purchase #2 $60.52   6.228%   $64.2892   $64.29   $64.30   $64.30  
  Purchase #3 $118.87   6.676%   $126.8058   $126.81   $126.80   $126.80  
  Purchase #4 $45.28   8.558%   $49.1551   $49.16   $49.15   $49.20  
  Purchase #5 $56.45   10.602%   $62.4348   $62.43   $62.45   $62.40  
  Purchase #6 $184.98   7.653%   $199.1365   $199.14   $199.15   $199.10  
  Purchase #7 $241.35   11.418%   $268.9073   $268.91   $268.90   $268.90  
  7 purchase totals = $731.88       $798.1983   $798.21   $798.20   $798.20