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 $20.74   4.502%   $21.6737   $21.67   $21.65   $21.70  
  Purchase #2 $55.95   13.543%   $63.5273   $63.53   $63.55   $63.50  
  Purchase #3 $172.59   4.596%   $180.5222   $180.52   $180.50   $180.50  
  Purchase #4 $5.68   12.821%   $6.4082   $6.41   $6.40   $6.40  
  Purchase #5 $230.62   11.533%   $257.2174   $257.22   $257.20   $257.20  
  Purchase #6 $169.33   8.638%   $183.9567   $183.96   $183.95   $184.00  
  Purchase #7 $117.39   3.441%   $121.4294   $121.43   $121.45   $121.40  
  7 purchase totals = $772.30       $834.7349   $834.74   $834.70   $834.70