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 $21.45   7.793%   $23.1216   $23.12   $23.10   $23.10  
  Purchase #2 $120.90   10.634%   $133.7565   $133.76   $133.75   $133.80  
  Purchase #3 $245.65   4.273%   $256.1466   $256.15   $256.15   $256.10  
  Purchase #4 $239.60   7.673%   $257.9845   $257.98   $258.00   $258.00  
  Purchase #5 $129.58   4.480%   $135.3852   $135.39   $135.40   $135.40  
  Purchase #6 $63.90   12.408%   $71.8287   $71.83   $71.85   $71.80  
  Purchase #7 $65.53   6.149%   $69.5594   $69.56   $69.55   $69.60  
  7 purchase totals = $886.61       $947.7825   $947.79   $947.80   $947.80