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 $15.87   11.400%   $17.6792   $17.68   $17.70   $17.70  
  Purchase #2 $7.40   5.329%   $7.7943   $7.79   $7.80   $7.80  
  Purchase #3 $133.55   9.604%   $146.3761   $146.38   $146.40   $146.40  
  Purchase #4 $13.94   10.410%   $15.3912   $15.39   $15.40   $15.40  
  Purchase #5 $57.88   13.760%   $65.8443   $65.84   $65.85   $65.80  
  Purchase #6 $153.86   13.616%   $174.8096   $174.81   $174.80   $174.80  
  Purchase #7 $25.30   8.315%   $27.4037   $27.40   $27.40   $27.40  
  7 purchase totals = $407.80       $455.2984   $455.29   $455.35   $455.30