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 $55.83   5.826%   $59.0827   $59.08   $59.10   $59.10  
  Purchase #2 $45.94   6.611%   $48.9771   $48.98   $49.00   $49.00  
  Purchase #3 $97.34   7.627%   $104.7641   $104.76   $104.75   $104.80  
  Purchase #4 $17.96   7.605%   $19.3259   $19.33   $19.35   $19.30  
  Purchase #5 $62.34   4.646%   $65.2363   $65.24   $65.25   $65.20  
  Purchase #6 $12.91   8.479%   $14.0046   $14.00   $14.00   $14.00  
  Purchase #7 $222.28   11.689%   $248.2623   $248.26   $248.25   $248.30  
  7 purchase totals = $514.60       $559.6530   $559.65   $559.70   $559.70