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 $246.57   12.189%   $276.6244   $276.62   $276.60   $276.60  
  Purchase #2 $159.79   13.841%   $181.9065   $181.91   $181.90   $181.90  
  Purchase #3 $75.78   8.659%   $82.3418   $82.34   $82.35   $82.30  
  Purchase #4 $103.89   6.324%   $110.46   $110.46   $110.45   $110.50  
  Purchase #5 $220.87   3.799%   $229.2609   $229.26   $229.25   $229.30  
  Purchase #6 $54.50   10.115%   $60.0127   $60.01   $60.00   $60.00  
  Purchase #7 $53.26   8.435%   $57.7525   $57.75   $57.75   $57.80  
  7 purchase totals = $914.66       $998.3588   $998.35   $998.30   $998.40