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 $18.31   9.186%   $19.992   $19.99   $20.00   $20.00  
  Purchase #2 $129.93   6.138%   $137.9051   $137.91   $137.90   $137.90  
  Purchase #3 $189.32   11.272%   $210.6602   $210.66   $210.65   $210.70  
  Purchase #4 $186.96   9.557%   $204.8278   $204.83   $204.85   $204.80  
  Purchase #5 $57.63   9.349%   $63.0178   $63.02   $63.00   $63.00  
  Purchase #6 $128.64   9.177%   $140.4453   $140.45   $140.45   $140.40  
  Purchase #7 $124.29   9.213%   $135.7408   $135.74   $135.75   $135.70  
  7 purchase totals = $835.08       $912.5890   $912.60   $912.60   $912.50