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 $62.46   6.352%   $66.4275   $66.43   $66.45   $66.40  
  Purchase #2 $39.59   5.625%   $41.8169   $41.82   $41.80   $41.80  
  Purchase #3 $135.72   4.424%   $141.7243   $141.72   $141.70   $141.70  
  Purchase #4 $158.29   13.761%   $180.0723   $180.07   $180.05   $180.10  
  Purchase #5 $247.90   8.556%   $269.1103   $269.11   $269.10   $269.10  
  Purchase #6 $185.79   3.519%   $192.328   $192.33   $192.35   $192.30  
  Purchase #7 $44.60   5.268%   $46.9495   $46.95   $46.95   $46.90  
  7 purchase totals = $874.35       $938.4288   $938.43   $938.40   $938.30