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 $242.99   12.150%   $272.5133   $272.51   $272.50   $272.50  
  Purchase #2 $223.77   3.203%   $230.9374   $230.94   $230.95   $230.90  
  Purchase #3 $126.70   13.256%   $143.4954   $143.50   $143.50   $143.50  
  Purchase #4 $47.60   10.811%   $52.746   $52.75   $52.75   $52.70  
  Purchase #5 $228.82   12.747%   $257.9877   $257.99   $258.00   $258.00  
  Purchase #6 $36.74   12.433%   $41.3079   $41.31   $41.30   $41.30  
  Purchase #7 $21.45   6.108%   $22.7602   $22.76   $22.75   $22.80  
  7 purchase totals = $928.07       $1,021.7479   $1,021.76   $1,021.75   $1,021.70