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 $99.65   13.815%   $113.4166   $113.42   $113.40   $113.40  
  Purchase #2 $194.38   9.725%   $213.2835   $213.28   $213.30   $213.30  
  Purchase #3 $180.50   8.494%   $195.8317   $195.83   $195.85   $195.80  
  Purchase #4 $57.73   10.190%   $63.6127   $63.61   $63.60   $63.60  
  Purchase #5 $12.71   12.469%   $14.2948   $14.29   $14.30   $14.30  
  Purchase #6 $28.27   11.501%   $31.5213   $31.52   $31.50   $31.50  
  Purchase #7 $6.71   7.772%   $7.2315   $7.23   $7.25   $7.20  
  7 purchase totals = $579.95       $639.1921   $639.18   $639.20   $639.10