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 $123.41   13.272%   $139.789   $139.79   $139.80   $139.80  
  Purchase #2 $211.84   12.130%   $237.5362   $237.54   $237.55   $237.50  
  Purchase #3 $137.57   6.671%   $146.7473   $146.75   $146.75   $146.70  
  Purchase #4 $123.89   9.604%   $135.7884   $135.79   $135.80   $135.80  
  Purchase #5 $63.94   8.753%   $69.5367   $69.54   $69.55   $69.50  
  Purchase #6 $175.97   11.301%   $195.8564   $195.86   $195.85   $195.90  
  Purchase #7 $158.69   6.671%   $169.2762   $169.28   $169.30   $169.30  
  7 purchase totals = $995.31       $1,094.5302   $1,094.55   $1,094.60   $1,094.50