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 $67.51   11.407%   $75.2109   $75.21   $75.20   $75.20  
  Purchase #2 $96.75   10.303%   $106.7182   $106.72   $106.70   $106.70  
  Purchase #3 $125.96   5.820%   $133.2909   $133.29   $133.30   $133.30  
  Purchase #4 $84.57   6.442%   $90.018   $90.02   $90.00   $90.00  
  Purchase #5 $145.92   10.119%   $160.6856   $160.69   $160.70   $160.70  
  Purchase #6 $17.31   12.861%   $19.5362   $19.54   $19.55   $19.50  
  Purchase #7 $236.96   7.348%   $254.3718   $254.37   $254.35   $254.40  
  7 purchase totals = $774.98       $839.8316   $839.84   $839.80   $839.80