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 $168.66   12.342%   $189.476   $189.48   $189.50   $189.50  
  Purchase #2 $93.42   4.647%   $97.7612   $97.76   $97.75   $97.80  
  Purchase #3 $149.68   13.352%   $169.6653   $169.67   $169.65   $169.70  
  Purchase #4 $34.88   11.557%   $38.9111   $38.91   $38.90   $38.90  
  Purchase #5 $85.53   4.785%   $89.6226   $89.62   $89.60   $89.60  
  Purchase #6 $203.36   7.772%   $219.1651   $219.17   $219.15   $219.20  
  Purchase #7 $218.30   3.516%   $225.9754   $225.98   $226.00   $226.00  
  7 purchase totals = $953.83       $1,030.5767   $1,030.59   $1,030.55   $1,030.70