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 $48.58   12.823%   $54.8094   $54.81   $54.80   $54.80  
  Purchase #2 $129.91   13.269%   $147.1478   $147.15   $147.15   $147.10  
  Purchase #3 $228.87   13.190%   $259.058   $259.06   $259.05   $259.10  
  Purchase #4 $136.31   5.315%   $143.5549   $143.55   $143.55   $143.60  
  Purchase #5 $52.42   13.784%   $59.6456   $59.65   $59.65   $59.60  
  Purchase #6 $141.94   8.185%   $153.5578   $153.56   $153.55   $153.60  
  Purchase #7 $16.93   5.546%   $17.8689   $17.87   $17.85   $17.90  
  7 purchase totals = $754.96       $835.6424   $835.65   $835.60   $835.70