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 $233.84   11.171%   $259.9623   $259.96   $259.95   $260.00  
  Purchase #2 $152.78   12.156%   $171.3519   $171.35   $171.35   $171.40  
  Purchase #3 $122.64   12.142%   $137.5309   $137.53   $137.55   $137.50  
  Purchase #4 $100.74   12.276%   $113.1068   $113.11   $113.10   $113.10  
  Purchase #5 $54.69   10.587%   $60.48   $60.48   $60.50   $60.50  
  Purchase #6 $191.79   10.525%   $211.9759   $211.98   $212.00   $212.00  
  Purchase #7 $29.59   11.355%   $32.9499   $32.95   $32.95   $32.90  
  7 purchase totals = $886.07       $987.3577   $987.36   $987.40   $987.40