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 $10.37   4.345%   $10.8206   $10.82   $10.80   $10.80  
  Purchase #2 $47.39   7.208%   $50.8059   $50.81   $50.80   $50.80  
  Purchase #3 $26.45   13.243%   $29.9528   $29.95   $29.95   $30.00  
  Purchase #4 $92.54   10.439%   $102.2003   $102.20   $102.20   $102.20  
  Purchase #5 $110.77   10.562%   $122.4695   $122.47   $122.45   $122.50  
  Purchase #6 $107.94   3.238%   $111.4351   $111.44   $111.45   $111.40  
  Purchase #7 $93.44   8.733%   $101.6001   $101.60   $101.60   $101.60  
  7 purchase totals = $488.90       $529.2843   $529.29   $529.25   $529.30