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 $79.84   12.609%   $89.907   $89.91   $89.90   $89.90  
  Purchase #2 $218.71   6.125%   $232.106   $232.11   $232.10   $232.10  
  Purchase #3 $16.51   9.142%   $18.0193   $18.02   $18.00   $18.00  
  Purchase #4 $12.46   3.312%   $12.8727   $12.87   $12.85   $12.90  
  Purchase #5 $169.50   6.379%   $180.3124   $180.31   $180.30   $180.30  
  Purchase #6 $160.50   8.196%   $173.6546   $173.65   $173.65   $173.70  
  Purchase #7 $213.58   4.446%   $223.0758   $223.08   $223.10   $223.10  
  7 purchase totals = $871.10       $929.9478   $929.95   $929.90   $930.00