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 $226.71   13.610%   $257.5652   $257.57   $257.55   $257.60  
  Purchase #2 $8.25   9.220%   $9.0107   $9.01   $9.00   $9.00  
  Purchase #3 $148.48   8.779%   $161.5151   $161.52   $161.50   $161.50  
  Purchase #4 $82.83   10.548%   $91.5669   $91.57   $91.55   $91.60  
  Purchase #5 $94.84   7.495%   $101.9483   $101.95   $101.95   $101.90  
  Purchase #6 $218.61   11.862%   $244.5415   $244.54   $244.55   $244.50  
  Purchase #7 $203.52   11.181%   $226.2756   $226.28   $226.30   $226.30  
  7 purchase totals = $983.24       $1,092.4233   $1,092.44   $1,092.40   $1,092.40