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 $232.78   3.140%   $240.0893   $240.09   $240.10   $240.10  
  Purchase #2 $242.45   13.703%   $275.6729   $275.67   $275.65   $275.70  
  Purchase #3 $207.52   9.114%   $226.4334   $226.43   $226.45   $226.40  
  Purchase #4 $32.62   11.848%   $36.4848   $36.48   $36.50   $36.50  
  Purchase #5 $189.47   13.457%   $214.967   $214.97   $214.95   $215.00  
  Purchase #6 $197.96   11.138%   $220.0088   $220.01   $220.00   $220.00  
  Purchase #7 $29.74   10.796%   $32.9507   $32.95   $32.95   $33.00  
  7 purchase totals = $1,132.54       $1,246.6069   $1,246.60   $1,246.60   $1,246.70