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 $49.74   3.208%   $51.3357   $51.34   $51.35   $51.30  
  Purchase #2 $188.29   10.285%   $207.6556   $207.66   $207.65   $207.70  
  Purchase #3 $151.71   4.413%   $158.405   $158.41   $158.40   $158.40  
  Purchase #4 $35.35   9.222%   $38.61   $38.61   $38.60   $38.60  
  Purchase #5 $235.67   8.486%   $255.669   $255.67   $255.65   $255.70  
  Purchase #6 $218.51   11.842%   $244.386   $244.39   $244.40   $244.40  
  Purchase #7 $62.34   4.486%   $65.1366   $65.14   $65.15   $65.10  
  7 purchase totals = $941.61       $1,021.1979   $1,021.22   $1,021.20   $1,021.20