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 $15.64   6.299%   $16.6252   $16.63   $16.65   $16.60  
  Purchase #2 $189.78   12.565%   $213.6259   $213.63   $213.65   $213.60  
  Purchase #3 $191.33   11.157%   $212.6767   $212.68   $212.70   $212.70  
  Purchase #4 $15.67   10.146%   $17.2599   $17.26   $17.25   $17.30  
  Purchase #5 $180.82   12.252%   $202.9741   $202.97   $202.95   $203.00  
  Purchase #6 $181.40   6.226%   $192.694   $192.69   $192.70   $192.70  
  Purchase #7 $156.91   5.643%   $165.7644   $165.76   $165.75   $165.80  
  7 purchase totals = $931.55       $1,021.6202   $1,021.62   $1,021.65   $1,021.70