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 $172.89   9.461%   $189.2471   $189.25   $189.25   $189.20  
  Purchase #2 $105.98   10.736%   $117.358   $117.36   $117.35   $117.40  
  Purchase #3 $9.57   3.418%   $9.8971   $9.90   $9.90   $9.90  
  Purchase #4 $73.43   7.279%   $78.775   $78.78   $78.80   $78.80  
  Purchase #5 $173.71   3.267%   $179.3851   $179.39   $179.40   $179.40  
  Purchase #6 $45.43   13.775%   $51.688   $51.69   $51.70   $51.70  
  Purchase #7 $223.49   13.617%   $253.9226   $253.92   $253.90   $253.90  
  7 purchase totals = $804.50       $880.2729   $880.29   $880.30   $880.30