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 $171.40   12.621%   $193.0324   $193.03   $193.05   $193.00  
  Purchase #2 $42.28   3.437%   $43.7332   $43.73   $43.75   $43.70  
  Purchase #3 $105.44   10.342%   $116.3446   $116.34   $116.35   $116.30  
  Purchase #4 $102.30   5.854%   $108.2886   $108.29   $108.30   $108.30  
  Purchase #5 $99.62   11.423%   $110.9996   $111.00   $111.00   $111.00  
  Purchase #6 $15.73   3.581%   $16.2933   $16.29   $16.30   $16.30  
  Purchase #7 $51.58   4.345%   $53.8212   $53.82   $53.80   $53.80  
  7 purchase totals = $588.35       $642.5129   $642.50   $642.55   $642.40