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 $113.64   6.829%   $121.4005   $121.40   $121.40   $121.40  
  Purchase #2 $7.59   3.214%   $7.8339   $7.83   $7.85   $7.80  
  Purchase #3 $123.60   4.521%   $129.188   $129.19   $129.20   $129.20  
  Purchase #4 $64.42   12.861%   $72.7051   $72.71   $72.70   $72.70  
  Purchase #5 $21.75   7.502%   $23.3817   $23.38   $23.40   $23.40  
  Purchase #6 $178.84   9.383%   $195.6206   $195.62   $195.60   $195.60  
  Purchase #7 $92.85   12.693%   $104.6355   $104.64   $104.65   $104.60  
  7 purchase totals = $602.69       $654.7653   $654.77   $654.80   $654.70