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 $34.34   12.838%   $38.7486   $38.75   $38.75   $38.70  
  Purchase #2 $245.43   11.358%   $273.3059   $273.31   $273.30   $273.30  
  Purchase #3 $155.26   9.472%   $169.9662   $169.97   $169.95   $170.00  
  Purchase #4 $29.85   11.399%   $33.2526   $33.25   $33.25   $33.30  
  Purchase #5 $31.95   6.424%   $34.0025   $34.00   $34.00   $34.00  
  Purchase #6 $174.43   4.355%   $182.0264   $182.03   $182.05   $182.00  
  Purchase #7 $241.31   6.279%   $256.4619   $256.46   $256.45   $256.50  
  7 purchase totals = $912.57       $987.7641   $987.77   $987.75   $987.80