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 $243.96   10.643%   $269.9247   $269.92   $269.90   $269.90  
  Purchase #2 $240.50   10.175%   $264.9709   $264.97   $264.95   $265.00  
  Purchase #3 $11.64   9.441%   $12.7389   $12.74   $12.75   $12.70  
  Purchase #4 $107.78   9.615%   $118.143   $118.14   $118.15   $118.10  
  Purchase #5 $26.61   3.402%   $27.5153   $27.52   $27.50   $27.50  
  Purchase #6 $70.33   4.199%   $73.2832   $73.28   $73.30   $73.30  
  Purchase #7 $68.76   9.835%   $75.5225   $75.52   $75.50   $75.50  
  7 purchase totals = $769.58       $842.0985   $842.09   $842.05   $842.00