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 $28.79   5.400%   $30.3447   $30.34   $30.35   $30.30  
  Purchase #2 $9.36   3.841%   $9.7195   $9.72   $9.70   $9.70  
  Purchase #3 $245.81   9.251%   $268.5499   $268.55   $268.55   $268.50  
  Purchase #4 $226.68   12.293%   $254.5458   $254.55   $254.55   $254.50  
  Purchase #5 $165.88   4.872%   $173.9617   $173.96   $173.95   $174.00  
  Purchase #6 $121.84   8.689%   $132.4267   $132.43   $132.45   $132.40  
  Purchase #7 $211.28   4.331%   $220.4305   $220.43   $220.45   $220.40  
  7 purchase totals = $1,009.64       $1,089.9788   $1,089.98   $1,090.00   $1,089.80