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 $20.68   5.702%   $21.8592   $21.86   $21.85   $21.90  
  Purchase #2 $161.44   12.640%   $181.846   $181.85   $181.85   $181.80  
  Purchase #3 $168.93   4.371%   $176.3139   $176.31   $176.30   $176.30  
  Purchase #4 $41.81   6.828%   $44.6648   $44.66   $44.65   $44.70  
  Purchase #5 $6.79   12.305%   $7.6255   $7.63   $7.65   $7.60  
  Purchase #6 $167.30   8.588%   $181.6677   $181.67   $181.65   $181.70  
  Purchase #7 $26.39   11.818%   $29.5088   $29.51   $29.50   $29.50  
  7 purchase totals = $593.34       $643.4859   $643.49   $643.45   $643.50