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 $227.92   9.696%   $250.0191   $250.02   $250.00   $250.00  
  Purchase #2 $137.32   11.751%   $153.4565   $153.46   $153.45   $153.50  
  Purchase #3 $188.88   13.733%   $214.8189   $214.82   $214.80   $214.80  
  Purchase #4 $126.52   9.278%   $138.2585   $138.26   $138.25   $138.30  
  Purchase #5 $38.78   6.527%   $41.3112   $41.31   $41.30   $41.30  
  Purchase #6 $221.31   9.186%   $241.6395   $241.64   $241.65   $241.60  
  Purchase #7 $205.30   12.395%   $230.7469   $230.75   $230.75   $230.70  
  7 purchase totals = $1,146.03       $1,270.2506   $1,270.26   $1,270.20   $1,270.20