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 $19.98   4.331%   $20.8453   $20.85   $20.85   $20.80  
  Purchase #2 $135.70   4.450%   $141.7387   $141.74   $141.75   $141.70  
  Purchase #3 $67.51   3.552%   $69.908   $69.91   $69.90   $69.90  
  Purchase #4 $121.53   4.386%   $126.8603   $126.86   $126.85   $126.90  
  Purchase #5 $118.28   12.518%   $133.0863   $133.09   $133.10   $133.10  
  Purchase #6 $179.30   5.689%   $189.5004   $189.50   $189.50   $189.50  
  Purchase #7 $148.93   8.331%   $161.3374   $161.34   $161.35   $161.30  
  7 purchase totals = $791.23       $843.2764   $843.29   $843.30   $843.20