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 $211.47   3.171%   $218.1757   $218.18   $218.20   $218.20  
  Purchase #2 $113.30   5.618%   $119.6652   $119.67   $119.65   $119.70  
  Purchase #3 $30.62   12.107%   $34.3272   $34.33   $34.35   $34.30  
  Purchase #4 $67.77   7.583%   $72.909   $72.91   $72.90   $72.90  
  Purchase #5 $249.43   11.830%   $278.9376   $278.94   $278.95   $278.90  
  Purchase #6 $171.86   8.119%   $185.8133   $185.81   $185.80   $185.80  
  Purchase #7 $104.30   5.803%   $110.3525   $110.35   $110.35   $110.40  
  7 purchase totals = $948.75       $1,020.1805   $1,020.19   $1,020.20   $1,020.20