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 $9.45   3.586%   $9.7889   $9.79   $9.80   $9.80  
  Purchase #2 $39.30   3.681%   $40.7466   $40.75   $40.75   $40.70  
  Purchase #3 $223.86   8.431%   $242.7336   $242.73   $242.75   $242.70  
  Purchase #4 $178.45   9.131%   $194.7443   $194.74   $194.75   $194.70  
  Purchase #5 $157.70   7.853%   $170.0842   $170.08   $170.10   $170.10  
  Purchase #6 $169.62   11.222%   $188.6548   $188.65   $188.65   $188.70  
  Purchase #7 $86.91   4.238%   $90.5932   $90.59   $90.60   $90.60  
  7 purchase totals = $865.29       $937.3456   $937.33   $937.40   $937.30