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 $14.76   13.724%   $16.7857   $16.79   $16.80   $16.80  
  Purchase #2 $209.71   4.866%   $219.9145   $219.91   $219.90   $219.90  
  Purchase #3 $20.68   7.851%   $22.3036   $22.30   $22.30   $22.30  
  Purchase #4 $119.57   4.751%   $125.2508   $125.25   $125.25   $125.30  
  Purchase #5 $152.63   7.757%   $164.4695   $164.47   $164.45   $164.50  
  Purchase #6 $221.39   9.612%   $242.67   $242.67   $242.65   $242.70  
  Purchase #7 $203.31   13.213%   $230.1734   $230.17   $230.15   $230.20  
  7 purchase totals = $942.05       $1,021.5675   $1,021.56   $1,021.50   $1,021.70