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 $237.98   13.659%   $270.4857   $270.49   $270.50   $270.50  
  Purchase #2 $119.39   3.803%   $123.9304   $123.93   $123.95   $123.90  
  Purchase #3 $212.55   5.707%   $224.6802   $224.68   $224.70   $224.70  
  Purchase #4 $146.77   9.234%   $160.3227   $160.32   $160.30   $160.30  
  Purchase #5 $37.96   12.600%   $42.743   $42.74   $42.75   $42.70  
  Purchase #6 $128.45   6.141%   $136.3381   $136.34   $136.35   $136.30  
  Purchase #7 $182.77   10.615%   $202.171   $202.17   $202.15   $202.20  
  7 purchase totals = $1,065.87       $1,160.6711   $1,160.67   $1,160.70   $1,160.60