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 $17.37   8.829%   $18.9036   $18.90   $18.90   $18.90  
  Purchase #2 $98.99   3.844%   $102.7952   $102.80   $102.80   $102.80  
  Purchase #3 $144.36   6.259%   $153.3955   $153.40   $153.40   $153.40  
  Purchase #4 $226.27   9.454%   $247.6616   $247.66   $247.65   $247.70  
  Purchase #5 $20.74   7.808%   $22.3594   $22.36   $22.35   $22.40  
  Purchase #6 $113.79   5.778%   $120.3648   $120.36   $120.35   $120.40  
  Purchase #7 $167.79   9.623%   $183.9364   $183.94   $183.95   $183.90  
  7 purchase totals = $789.31       $849.4165   $849.42   $849.40   $849.50