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 $137.78   8.804%   $149.9102   $149.91   $149.90   $149.90  
  Purchase #2 $37.26   8.711%   $40.5057   $40.51   $40.50   $40.50  
  Purchase #3 $138.54   12.386%   $155.6996   $155.70   $155.70   $155.70  
  Purchase #4 $236.79   7.436%   $254.3977   $254.40   $254.40   $254.40  
  Purchase #5 $30.67   6.504%   $32.6648   $32.66   $32.65   $32.70  
  Purchase #6 $29.62   13.854%   $33.7236   $33.72   $33.70   $33.70  
  Purchase #7 $75.56   4.844%   $79.2201   $79.22   $79.20   $79.20  
  7 purchase totals = $686.22       $746.1217   $746.12   $746.05   $746.10