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 $17.74   6.362%   $18.8686   $18.87   $18.85   $18.90  
  Purchase #2 $146.80   11.529%   $163.7246   $163.72   $163.70   $163.70  
  Purchase #3 $77.65   6.412%   $82.6289   $82.63   $82.65   $82.60  
  Purchase #4 $131.58   13.507%   $149.3525   $149.35   $149.35   $149.40  
  Purchase #5 $70.86   13.695%   $80.5643   $80.56   $80.55   $80.60  
  Purchase #6 $100.79   9.402%   $110.2663   $110.27   $110.25   $110.30  
  Purchase #7 $107.53   7.453%   $115.5442   $115.54   $115.55   $115.50  
  7 purchase totals = $652.95       $720.9494   $720.94   $720.90   $721.00