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 $166.49   13.676%   $189.2592   $189.26   $189.25   $189.30  
  Purchase #2 $220.67   9.563%   $241.7727   $241.77   $241.75   $241.80  
  Purchase #3 $66.75   5.635%   $70.5114   $70.51   $70.50   $70.50  
  Purchase #4 $59.74   5.570%   $63.0675   $63.07   $63.05   $63.10  
  Purchase #5 $58.40   13.368%   $66.2069   $66.21   $66.20   $66.20  
  Purchase #6 $174.82   8.116%   $189.0084   $189.01   $189.00   $189.00  
  Purchase #7 $175.82   12.306%   $197.4564   $197.46   $197.45   $197.50  
  7 purchase totals = $922.69       $1,017.2825   $1,017.29   $1,017.20   $1,017.40