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 $39.97   5.449%   $42.148   $42.15   $42.15   $42.10  
  Purchase #2 $38.71   7.733%   $41.7034   $41.70   $41.70   $41.70  
  Purchase #3 $18.58   8.452%   $20.1504   $20.15   $20.15   $20.20  
  Purchase #4 $120.79   12.422%   $135.7945   $135.79   $135.80   $135.80  
  Purchase #5 $136.29   9.133%   $148.7374   $148.74   $148.75   $148.70  
  Purchase #6 $53.61   10.357%   $59.1624   $59.16   $59.15   $59.20  
  Purchase #7 $109.45   13.689%   $124.4326   $124.43   $124.45   $124.40  
  7 purchase totals = $517.40       $572.1287   $572.12   $572.15   $572.10