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 $58.78   12.822%   $66.3168   $66.32   $66.30   $66.30  
  Purchase #2 $236.37   9.118%   $257.9222   $257.92   $257.90   $257.90  
  Purchase #3 $59.47   10.714%   $65.8416   $65.84   $65.85   $65.80  
  Purchase #4 $76.36   5.226%   $80.3506   $80.35   $80.35   $80.40  
  Purchase #5 $128.50   6.621%   $137.008   $137.01   $137.00   $137.00  
  Purchase #6 $33.42   13.642%   $37.9792   $37.98   $38.00   $38.00  
  Purchase #7 $154.79   3.612%   $160.381   $160.38   $160.40   $160.40  
  7 purchase totals = $747.69       $805.7994   $805.80   $805.80   $805.80