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 $244.81   13.232%   $277.2033   $277.20   $277.20   $277.20  
  Purchase #2 $11.56   8.824%   $12.5801   $12.58   $12.60   $12.60  
  Purchase #3 $111.36   9.276%   $121.6898   $121.69   $121.70   $121.70  
  Purchase #4 $85.74   9.599%   $93.9702   $93.97   $93.95   $94.00  
  Purchase #5 $19.69   11.254%   $21.9059   $21.91   $21.90   $21.90  
  Purchase #6 $96.65   9.717%   $106.0415   $106.04   $106.05   $106.00  
  Purchase #7 $229.38   12.769%   $258.6695   $258.67   $258.65   $258.70  
  7 purchase totals = $799.19       $892.0603   $892.06   $892.05   $892.10