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 $243.64   12.422%   $273.905   $273.91   $273.90   $273.90  
  Purchase #2 $32.59   9.413%   $35.6577   $35.66   $35.65   $35.70  
  Purchase #3 $187.34   8.136%   $202.582   $202.58   $202.60   $202.60  
  Purchase #4 $150.67   10.291%   $166.1754   $166.18   $166.20   $166.20  
  Purchase #5 $169.59   11.402%   $188.9267   $188.93   $188.95   $188.90  
  Purchase #6 $237.62   6.572%   $253.2364   $253.24   $253.25   $253.20  
  Purchase #7 $219.96   6.757%   $234.8227   $234.82   $234.80   $234.80  
  7 purchase totals = $1,241.41       $1,355.3059   $1,355.32   $1,355.35   $1,355.30