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 $246.51   11.370%   $274.5382   $274.54   $274.55   $274.50  
  Purchase #2 $118.57   9.446%   $129.7701   $129.77   $129.75   $129.80  
  Purchase #3 $248.52   7.456%   $267.0497   $267.05   $267.05   $267.00  
  Purchase #4 $68.43   7.685%   $73.6888   $73.69   $73.70   $73.70  
  Purchase #5 $193.93   6.492%   $206.5199   $206.52   $206.50   $206.50  
  Purchase #6 $172.93   8.745%   $188.0527   $188.05   $188.05   $188.10  
  Purchase #7 $131.98   9.374%   $144.3518   $144.35   $144.35   $144.40  
  7 purchase totals = $1,180.87       $1,283.9712   $1,283.97   $1,283.95   $1,284.00