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 $242.93   3.722%   $251.9719   $251.97   $251.95   $252.00  
  Purchase #2 $51.52   13.315%   $58.3799   $58.38   $58.40   $58.40  
  Purchase #3 $12.41   8.446%   $13.4581   $13.46   $13.45   $13.50  
  Purchase #4 $91.35   4.463%   $95.427   $95.43   $95.45   $95.40  
  Purchase #5 $91.32   12.316%   $102.567   $102.57   $102.55   $102.60  
  Purchase #6 $94.38   10.499%   $104.289   $104.29   $104.30   $104.30  
  Purchase #7 $49.60   4.517%   $51.8404   $51.84   $51.85   $51.80  
  7 purchase totals = $633.51       $677.9333   $677.94   $677.95   $678.00