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 $158.79   12.597%   $178.7928   $178.79   $178.80   $178.80  
  Purchase #2 $130.93   5.483%   $138.1089   $138.11   $138.10   $138.10  
  Purchase #3 $75.51   10.571%   $83.4922   $83.49   $83.50   $83.50  
  Purchase #4 $50.90   11.625%   $56.8171   $56.82   $56.80   $56.80  
  Purchase #5 $43.52   9.192%   $47.5204   $47.52   $47.50   $47.50  
  Purchase #6 $155.75   9.691%   $170.8437   $170.84   $170.85   $170.80  
  Purchase #7 $139.84   13.748%   $159.0652   $159.07   $159.05   $159.10  
  7 purchase totals = $755.24       $834.6403   $834.64   $834.60   $834.60