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 $81.48   3.226%   $84.1085   $84.11   $84.10   $84.10  
  Purchase #2 $128.80   6.621%   $137.3278   $137.33   $137.35   $137.30  
  Purchase #3 $231.81   8.227%   $250.881   $250.88   $250.90   $250.90  
  Purchase #4 $191.28   4.262%   $199.4324   $199.43   $199.45   $199.40  
  Purchase #5 $121.84   11.218%   $135.508   $135.51   $135.50   $135.50  
  Purchase #6 $243.79   9.615%   $267.2304   $267.23   $267.25   $267.20  
  Purchase #7 $215.62   8.667%   $234.3078   $234.31   $234.30   $234.30  
  7 purchase totals = $1,214.62       $1,308.7959   $1,308.80   $1,308.85   $1,308.70