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 $80.84   11.103%   $89.8157   $89.82   $89.80   $89.80  
  Purchase #2 $175.78   7.795%   $189.4821   $189.48   $189.50   $189.50  
  Purchase #3 $22.25   6.126%   $23.613   $23.61   $23.60   $23.60  
  Purchase #4 $116.58   11.275%   $129.7244   $129.72   $129.70   $129.70  
  Purchase #5 $13.53   10.130%   $14.9006   $14.90   $14.90   $14.90  
  Purchase #6 $99.30   10.850%   $110.0741   $110.07   $110.05   $110.10  
  Purchase #7 $61.52   4.536%   $64.3105   $64.31   $64.30   $64.30  
  7 purchase totals = $569.80       $621.9204   $621.91   $621.85   $621.90