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 $240.96   13.287%   $272.9764   $272.98   $273.00   $273.00  
  Purchase #2 $83.40   11.290%   $92.8159   $92.82   $92.80   $92.80  
  Purchase #3 $54.47   6.448%   $57.9822   $57.98   $58.00   $58.00  
  Purchase #4 $55.68   5.572%   $58.7825   $58.78   $58.80   $58.80  
  Purchase #5 $148.48   3.553%   $153.7555   $153.76   $153.75   $153.80  
  Purchase #6 $178.56   6.861%   $190.811   $190.81   $190.80   $190.80  
  Purchase #7 $116.79   5.503%   $123.217   $123.22   $123.20   $123.20  
  7 purchase totals = $878.34       $950.3405   $950.35   $950.35   $950.40