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 $10.81   9.572%   $11.8447   $11.84   $11.85   $11.80  
  Purchase #2 $7.92   6.817%   $8.4599   $8.46   $8.45   $8.50  
  Purchase #3 $193.67   12.112%   $217.1273   $217.13   $217.15   $217.10  
  Purchase #4 $145.87   9.153%   $159.2215   $159.22   $159.20   $159.20  
  Purchase #5 $21.39   9.445%   $23.4103   $23.41   $23.40   $23.40  
  Purchase #6 $56.28   6.331%   $59.8431   $59.84   $59.85   $59.80  
  Purchase #7 $142.36   12.680%   $160.4112   $160.41   $160.40   $160.40  
  7 purchase totals = $578.30       $640.3180   $640.31   $640.30   $640.20