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 $29.40   6.679%   $31.3636   $31.36   $31.35   $31.40  
  Purchase #2 $201.27   10.191%   $221.7814   $221.78   $221.80   $221.80  
  Purchase #3 $97.39   11.836%   $108.9171   $108.92   $108.90   $108.90  
  Purchase #4 $99.83   10.676%   $110.4879   $110.49   $110.50   $110.50  
  Purchase #5 $105.69   13.833%   $120.3101   $120.31   $120.30   $120.30  
  Purchase #6 $9.31   8.176%   $10.0712   $10.07   $10.05   $10.10  
  Purchase #7 $31.92   7.812%   $34.4136   $34.41   $34.40   $34.40  
  7 purchase totals = $574.81       $637.3449   $637.34   $637.30   $637.40