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 $9.61   12.321%   $10.794   $10.79   $10.80   $10.80  
  Purchase #2 $19.65   5.704%   $20.7708   $20.77   $20.75   $20.80  
  Purchase #3 $11.33   13.639%   $12.8753   $12.88   $12.90   $12.90  
  Purchase #4 $162.77   9.707%   $178.5701   $178.57   $178.55   $178.60  
  Purchase #5 $206.63   6.288%   $219.6229   $219.62   $219.60   $219.60  
  Purchase #6 $12.75   7.734%   $13.7361   $13.74   $13.75   $13.70  
  Purchase #7 $185.85   12.795%   $209.6295   $209.63   $209.65   $209.60  
  7 purchase totals = $608.59       $665.9987   $666.00   $666.00   $666.00