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 $27.37   12.765%   $30.8638   $30.86   $30.85   $30.90  
  Purchase #2 $23.50   12.615%   $26.4645   $26.46   $26.45   $26.50  
  Purchase #3 $122.30   6.333%   $130.0453   $130.05   $130.05   $130.00  
  Purchase #4 $134.48   5.297%   $141.6034   $141.60   $141.60   $141.60  
  Purchase #5 $96.84   11.460%   $107.9379   $107.94   $107.95   $107.90  
  Purchase #6 $140.54   4.598%   $147.002   $147.00   $147.00   $147.00  
  Purchase #7 $63.98   13.329%   $72.5079   $72.51   $72.50   $72.50  
  7 purchase totals = $609.01       $656.4248   $656.42   $656.40   $656.40