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 $231.87   13.663%   $263.5504   $263.55   $263.55   $263.60  
  Purchase #2 $102.55   8.757%   $111.5303   $111.53   $111.55   $111.50  
  Purchase #3 $244.31   11.754%   $273.0262   $273.03   $273.05   $273.00  
  Purchase #4 $162.91   3.775%   $169.0599   $169.06   $169.05   $169.10  
  Purchase #5 $157.51   12.141%   $176.6333   $176.63   $176.65   $176.60  
  Purchase #6 $107.96   11.476%   $120.3495   $120.35   $120.35   $120.30  
  Purchase #7 $160.69   5.399%   $169.3657   $169.37   $169.35   $169.40  
  7 purchase totals = $1,167.80       $1,283.5153   $1,283.52   $1,283.55   $1,283.50