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 $61.34   10.519%   $67.7924   $67.79   $67.80   $67.80  
  Purchase #2 $59.46   8.545%   $64.5409   $64.54   $64.55   $64.50  
  Purchase #3 $55.72   3.211%   $57.5092   $57.51   $57.50   $57.50  
  Purchase #4 $243.33   5.716%   $257.2387   $257.24   $257.25   $257.20  
  Purchase #5 $175.27   12.638%   $197.4206   $197.42   $197.40   $197.40  
  Purchase #6 $180.26   11.568%   $201.1125   $201.11   $201.10   $201.10  
  Purchase #7 $14.88   6.259%   $15.8113   $15.81   $15.80   $15.80  
  7 purchase totals = $790.26       $861.4256   $861.42   $861.40   $861.30