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 $12.37   4.216%   $12.8915   $12.89   $12.90   $12.90  
  Purchase #2 $145.68   7.770%   $156.9993   $157.00   $157.00   $157.00  
  Purchase #3 $211.73   3.125%   $218.3466   $218.35   $218.35   $218.30  
  Purchase #4 $42.71   6.724%   $45.5818   $45.58   $45.60   $45.60  
  Purchase #5 $189.92   8.180%   $205.4555   $205.46   $205.45   $205.50  
  Purchase #6 $218.61   9.154%   $238.6216   $238.62   $238.60   $238.60  
  Purchase #7 $48.33   7.270%   $51.8436   $51.84   $51.85   $51.80  
  7 purchase totals = $869.35       $929.7399   $929.74   $929.75   $929.70