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 $178.54   4.318%   $186.2494   $186.25   $186.25   $186.20  
  Purchase #2 $160.92   9.331%   $175.9354   $175.94   $175.95   $175.90  
  Purchase #3 $152.37   13.872%   $173.5068   $173.51   $173.50   $173.50  
  Purchase #4 $115.35   5.323%   $121.4901   $121.49   $121.50   $121.50  
  Purchase #5 $236.64   13.563%   $268.7355   $268.74   $268.75   $268.70  
  Purchase #6 $60.90   5.562%   $64.2873   $64.29   $64.30   $64.30  
  Purchase #7 $26.67   7.378%   $28.6377   $28.64   $28.65   $28.60  
  7 purchase totals = $931.39       $1,018.8422   $1,018.86   $1,018.90   $1,018.70