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 $29.45   11.638%   $32.8774   $32.88   $32.90   $32.90  
  Purchase #2 $226.65   12.264%   $254.4464   $254.45   $254.45   $254.40  
  Purchase #3 $88.56   11.431%   $98.6833   $98.68   $98.70   $98.70  
  Purchase #4 $187.38   11.661%   $209.2304   $209.23   $209.25   $209.20  
  Purchase #5 $228.56   12.772%   $257.7517   $257.75   $257.75   $257.80  
  Purchase #6 $224.59   3.602%   $232.6797   $232.68   $232.70   $232.70  
  Purchase #7 $177.76   9.276%   $194.249   $194.25   $194.25   $194.20  
  7 purchase totals = $1,162.95       $1,279.9179   $1,279.92   $1,280.00   $1,279.90