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 $249.95   13.512%   $283.7232   $283.72   $283.70   $283.70  
  Purchase #2 $91.45   3.557%   $94.7029   $94.70   $94.70   $94.70  
  Purchase #3 $211.28   13.422%   $239.638   $239.64   $239.65   $239.60  
  Purchase #4 $161.94   4.849%   $169.7925   $169.79   $169.80   $169.80  
  Purchase #5 $55.25   5.249%   $58.1501   $58.15   $58.15   $58.20  
  Purchase #6 $128.48   3.405%   $132.8547   $132.85   $132.85   $132.90  
  Purchase #7 $12.67   4.317%   $13.217   $13.22   $13.20   $13.20  
  7 purchase totals = $911.02       $992.0784   $992.07   $992.05   $992.10