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 $62.84   6.121%   $66.6864   $66.69   $66.70   $66.70  
  Purchase #2 $101.55   9.345%   $111.0398   $111.04   $111.05   $111.00  
  Purchase #3 $222.95   10.669%   $246.7365   $246.74   $246.75   $246.70  
  Purchase #4 $65.89   11.583%   $73.522   $73.52   $73.50   $73.50  
  Purchase #5 $150.35   8.211%   $162.6952   $162.70   $162.70   $162.70  
  Purchase #6 $47.77   7.802%   $51.497   $51.50   $51.50   $51.50  
  Purchase #7 $201.73   12.438%   $226.8212   $226.82   $226.80   $226.80  
  7 purchase totals = $853.08       $938.9981   $939.01   $939.00   $938.90